Luton and EasyJet

It is little more difficult to get to since it is only served directly via bus. We took the bus from the National bus terminal near Victoria station to the airport. Seem to remember it was a bout a 30, maybe 45 min run to the airport. The airport, itself, is small and easy to get around.

or train from St Pancras to Luton Airport Parkway then a shuttle bus

https://www.london-luton.co.uk/to-and-from-lla

The easiest way to get to Luton Airport is by train. Thameslink 3 times an hour on its service from south of London via Blackfriars, City Thameslink, Farringdon and St Pancras International to Bedford. East Midlands trains from St Pancras International which stop at Luton Airport Parkway (be careful, many don't, the twice hourly EMR to Corby does).

There will soon be a people mover from the station into the airport but while the finishing touches are being applied there is still a bus between the station and the terminal. About 10 minutes to add to train time, including loading and waiting. Get tickets to Luton Airport - then the bus is free - and not to Luton Airport Parkway. If your ticket is to Luton Airport Parkway you can pay for the bus, £2.40 return.

EDIT - see message from me 5 Jan below

Is Schiphol your final destination or are you travelling to Amsterdam?

If you actually want to go to Amsterdam, my thought is don't book this flight. Take the train instead, it departs from central London.

I landed there once, and it was easy to get to London, although there is some time waiting on an outdoor platform at Luton Airport Parkway so could be a pain in bad weather. From the airport its a regular shuttle bus to the train station. From Luton Airport Parkway there is very regular service to London St Pancras.

There will soon be a people mover from the station into the airport

The news tonight announced that the new Dart driverless train conveyance gizmo ridden last year by the King will operational and open to the public by Easter. That will replace the bus between the station and the terminal.

So the good news is, Alex, no bus for you - ride the Dart from the station to the terminal.

When you buy your rail ticket ask specifically for one to Luton Airport as that will include the cost of the bus (or people mover if it is open then) to the Airport. If you get a ticket to Luton Airport Parkway station you will need to buy a separate bus ticket to the actual airport.

The people mover is now open. Buy a ticket to Luton Airport not to Luton Airport Parkway, then it will include the people mover at a heavily discounted rate.
The journey from St Pancras to airport terminal is advertised as 32 minutes, and that's exactly what I did it in tonight for £5 inclusive of the people mover, advance fare on a railcard. That is where I am writing this from.
That fare is on East Midlands trains services, not Thameslink.
There is a new terminal, which is simplicity itself to get through- a vast improvement on the old.
The ATM's, to obtain cash in both pounds and euro are free to use.
The bus ended at the end of March, although there is still a local bus to Luton town 24 hours a day, which calls at the station.
The stand alone cost of the shuttle is £4.90

The ATM's, to obtain cash in both pounds and sterling are free to use.

Perhaps that’s £ and €? Or is their some distinction bet sterling and pounds that I’m not aware of?

OK, edited. I had less than 4 hours sleep last night, and will have even less tonight, so obviously it was catching up with me.
While waiting I noticed that at least one of the airport shops accepts Canadian, US and Australian dollars, even though it is not a long haul airport.

Why not take Eurostar to Amsterdam.?

If you go to St Pancras to get a train to go to Luton, why not just take the train from there to Amsterdam and avoid the airport mess and easyjets hoops and hurdles ?

"While waiting I noticed that at least one of the airport shops accepts Canadian, US and Australian dollars"

The Fex will be horrific and chances are no change will be given. Yes, Pounds and Sterling are one and the same.

Indeed the Eurostar is a good or perhaps even the better option. Though its likely EJ prices despite the palaver are cheaper and its amazing how much some will put up with to save a couple of quid.

The UK currency is technically pounds sterling.
A number of other countries around the world use pounds, most if not all former British colonies, now independent sovereign nations.
The OP did not ask for comments about his modal choice, nor did he deserve to be effectively called cheap.
There could be many reasons why the OP chooses to fly, such as Easy jet gets him there at a time when Eurostar can't. Whatever the reasons it is none of our business.
Personally I would take the train to most of the destinations served by Luton (except Tenerife and Iceland) on environmental grounds. But that is me. Most of those are eastern European destinations you've never heard of, served by Wizz. It takes multiple times longer, and multiple times the cost, but then you only travel if your journey is necessary, not just for the sake of it.
Sadly it is not realistically viable now to go by sea to North America, so flying is the only practicable option. Even when they can a West coast destination is near impossible on a line voyage. Even I have a very hard job justifying Amtrak's rates for transcontinental travel, purely as a point A to point B journey.

"A number of other countries around the world use pounds, most if not all former British colonies, now independent sovereign nations."

Err... hardly any do, have ever tried using Pounds in HongKong and Singapore?

Cheap and cheaper are entirely different, but you know that ;-)

The idea of travel fora is to offer alternatives, the OP is free to heed it, ignore it or distill it with other advice given.

are Pounds used in India, Pakistan, Bangladesh, Canada, USA, Hong Kong, Ireland, British Virgin Islands? Oh dear.

I'm pretty sure that "most if not all former British colonies" do not actually use Pound Sterling.

Read what was said, no-one said that all former British Colonies used pounds. What was said was that countries which do are probably all former Colonies.
In fact the actual answer turns out to be more interesting and nuanced-
Falkland Islands (their currency is also used in South Georgia and South Sandwich), Egypt, Gibraltar, Guernsey, Isle of Man, Jersey, Lebanon, South Sudan, Sudan and Syria.
The pound sterling is also used in Tristan da Cunha and the British Antarctic and British Indian Ocean Territories. And the pound sterling is also used interchangeably with the local pound in Gibraltar, Jersey, Guernsey and the Isle of Man.
I was not expecting to actually have to look this up, but the 'point scoring' of certain people has forced my hand. Instead of trying to point score and not reading what was written other people could have similarly googled this.

who is point scoring? What does this have to do with Easyjet?

There could be many reasons why the OP chooses to fly, such as Easy
jet gets him there at a time when Eurostar can't. Whatever the reasons
it is none of our business.

Or maybe they are just an american that have grown up in world where there are two ways go get around, drive or fly. I've seen so many examples of North Americans making weird travel choices because it just didn't occur to them that the train is an option. So I'm going to give opinions on modal choice when there is a better option in my opinion.

If I was going to Amsterdam from home I would in the past have, by choice, gone by ferry via Newcastle, Hull or Harwich. I am gaining a night by doing so. But I don't offer that choice on this forum for two reasons- firstly every time a ferry is mentioned you get these comments about "well it might be stormy and get cancelled". Ferries in the US (even in the PNW) are nothing like those in Europe. They are well stabilised and generally bigger and more luxurious. What often stops modern ferries isn't the sea conditions, it's berthing conditions at the ports. That is no different to planes being unable to land at airports due to severe weather.
But the second reason is how NS have made the onward trip from Hook of Holland harder- firstly the handing over of the heavy rail line to Rotterdam to be converted for "tram use" but secondly the withdrawal of the use of the inclusive ticket for use on NS.
It's just made it so more cumbersome and expensive.
I wouldn't use Eurostar, travelling from the north of England, simply because of the risk of a missed connection in London. Now we have the CIV ticket, maybe that makes a difference. Also interesting to see that the CIV ticket covers connections to Harwich for the Hook of Holland boat.
It is not only Americans who think "flight good, train bad". There wouldn't be so many flights to the Netherlands, Belgium, France and Germany if UK citizens were somehow more educated about the possibilities of high speed rail. And it was bizarre on Tuesday night how many people were spending the night at the airport (6 to 8 hours) just to get an early flight.
We need to convert our own citizens first before American visitors.

are Pounds used in India, Pakistan, Bangladesh, Canada, USA, Hong Kong, Ireland, British Virgin Islands? Oh dear. I'm pretty sure that "most if not all former British colonies" do not actually use Pound Sterling.
who is point scoring? What does this have to do with Easyjet?

Exactly little or nothing to do with Easy Jet. When someone writes the first comment, especially "Oh Dear" that is point scoring. It is ridiculous how many posts have had to do with one typo.
This is a travel forum, not facebook.
I actually thought some African countries still used a local pound, but on looking it up they have moved away at some stage from that.
Two people for some reason have brought up Hong Kong. I'm pretty sure that before Handover expats were able to spend pounds sterling there. It's a long time ago now, and certainly not so now. While we usually did spend HKD I don't remember that it was impossible to spend £ in the more touristy areas of HK Island, rather than in the New Territories or in the Out Islands. Just as HKD could be freely used on day trips to Macau, in fact were preferred.

Very easy from St Pancras and then using the new DART from the arriving station

We are traveling from Seattle to London on British Airways, arriving at 1:10 PM. (Next September, BTW)
We are saving money by purchasing a separate ticket from Heathrow to Amsterdam later that same day. We are booking on KLM. One flight departs at 2:45 PM and another at 4:05 PM.
https://community.ricksteves.com/travel-forum/general-europe/time-needed-at-lhr

An earlier post by OP.

Cannot contain my curiosity, but I wonder if OP is now contemplating transferring from LHR to LTN to continue on to AMS the same day.