Last day - Heathrow

According to scotrail it’s minimum of 4.5 hours and more likely 5hrs 45 min train ride to London. So even with a 9am train you won’t get in until early afternoon. Might be a little late to start heading to Windsor?

There’s a Premier inn near Kings Cross/St Pancras. I have stayed there but many forum members like the chain. Nearby is the treasure room at the British Library is worth a visit. What are your interests?

https://www.scotrail.co.uk/train-times/glasgow-to-london

Scotrail don't operate the trains to London, That is Avanti West Coast.

That information is only partly correct.

If you catch the hourly train at broadly half past the hour that takes 4 1/2 hours as it goes direct. The other train (which runs broadly every 2 hours) and leaves at around 50 minutes past each hour takes a longer route through Birmingham taking around 5 3/4 hours.

Say you get the 0830 ish from Glasgow that'll be 1pm into London, which would connect you to Windsor by 3pm. A bit late for the Castle.

You don't say what time your flight is but if it's morning as you say and you want to be there a minimum of 3 hours beforehand I'd be worried about staying outside the tube system the previous night.
So find a PI, and just do something in Central London, like the British Museum.
Maybe go to Sung Evensong at St Paul's, Westminster or any of a whole host of other Churches. Maybe an evening Thames cruise by either service boat (Uber by Thames Clippers) or dedicated cruise boat. Some of them do dinner cruises.

A standard double room at the Kings Cross Premier Inn will be £232 (flexible rate) on June 8, 15, or 22., without breakfast. That gives you an idea of what you can expect to pay for a budget-friendly hotel in London on a Thursday night in June.

I suggest you choose your hotel location based on ease and cost of getting to the airport on Friday, rather than some nearby attraction you want to take in. If your hotel is near a tube station you can easily go to any of the popular attractions for the afternoon and evening.

You will get lots of opinions on this, but our standard practice for the night before our flight home is to stay close to Paddington Station and book the low-cost advance tickets (£5.50) on the Heathrow Express for our ride to Heathrow the next day. The HEx is the fastest and most comfortable way to Heathrow, and takes you right into T3T2 or T5 in 15 or 20 minutes.

These must be booked 90+ days in advance, and they are limited in number. People have been reporting they are already sold out for some dates in summer, but I checked random Fridays in June and they are still available.

https://www.heathrowexpress.com/tickets-and-offers/book-early-to-save-money#/

So start by looking at these for your departure date, and if you can get the cheap tix then consider booking the new-ish Pilgrm Hotel close by Paddington( yes it is spelled that way).

https://thepilgrm.com/

Their Kingbed rooms are £229 in June, or a smaller double room is £219, both fully-refundable flexible rates, without breakfast. (I would choose the larger one).

When you arrive at Kings Cross station, it is an easy 10-minute tube ride from there to Paddington. Check in and leave your luggage, and then head out to explore London. This is just a few blocks north of Hyde Park, and you might like to stretch you legs with a walk in the park after that long train ride. You can walk through the park to reach some museums like the Victoria and Albert or the wonderful science museum, or the area around Buckingham Palace (a bit further away). The museums are free ( unless things have changed recently) and of course walking around is too.

Wow, 232 pounds for a Premier Inn! London hotel prices are giving me sticker shock.

It is slightly lower if you pay in advance for a non-refundable booking, but that is not a good idea.

London hotel prices went silly high in general from spring 2022 onwards and have had no signs of dropping back yet (demand, supply, workforce shortages, costs etc). Complete contrast with 2021 when they were really dirt cheap.

It’s already too late to get the cheap £5.50 fares on the HEX for June. You now have to book 5 months in advance.
If you’re happy just to wander round and ear/drink in Windsor then there’s nothing wrong with that as an option. It’s an easy quick taxi ride to Heathrow from there.

I notice in your original question you wondered about staying near Heathrow airport at a lower cost hotel. All the Premier Inns near Heathrow are less than $100 for the same night as was quoted for a Central London Hotel. It also gives you the advantage of being close to the airport for your morning departure.

I might be tempted stay near Heathrow and spend the afternoon/evening at Kew Gardens in Richmond which is an easy tube ride away.

I just checked the Heathrow Express site for two different Friday's in June and the 5.50 GBP tickets were available.

Frank II has confirmed what I found and reported above: the £5.50 Heathrow Express tickets are available for at least 2 of the Fridays in June. I did not check the others. I did see earlier that some summer dates (July) were sold out. But maybe they released more?

So that is an option. Windsor is also a nice choice (we did that once before flying home). But you may find hotel prices just as high there.

The Premier Inns at or near Heathrow ( there are several) are indeed under £100. But you should look carefully at the cost, time involved, and inconvenience of getting there from Kings Cross, after the long train ride, to check in and drop your luggage, and then heading into London (or Windsor) for sightseeing, then returning to the hotel after dinner to sleep. You are not going to want to spend the afternoon and evening sitting in a hotel at the airport.

Well HEX had raised the bar to 5 months as stated previously stated. Perhaps they have realised the commercial futility of such a decision.
I was looking just now at Thursday 15 June. There is a PI at Hayes, near Heathrow for as little as £66.50. It's six miles from the airport but there is a 24 hour bus route- the 140 stops outside.
Only a short distance from Hayes and Harlington Elizabeth line station.
Plenty of others in the same area at the same kind of price, with the same easy airport access.
Most of the off site airport hotels have very easy transit access into the airport. The very good local bus network is often overlooked, many of the routes are either 24 hour or run until after midnight/start before 4am- for airport workers. To save £150 it's totally worth it.

I would always go on the 205 bus from Kings Cross or Euston to Paddington. TfL give a 22 minute journey time, which is pretty accurate and consistent in my experience.

I see from the HEX website that it now says that you can book early to save and that could be as low as £5.50, but there is no longer a reference to how soon to get those cheapest tickets. They clearly have seen the pointlessness of making those prices harder to find when they have a new more flexible option in the Elizabeth line to compete with.

I've stayed several times at the Premier Inn Heathrow T4. It's connected to Terminal 4 via an enclosed walkway, but it's perhaps a 10-minute walk (my estimate). You can get from T4 to your departure terminal the next morning via the Underground. Rates refundable until 1 PM on day of arrival for the Thursdays in June are as follows;

June 1: £66
June 8: £94
June 15: £94
June 22: £94
June 29: £94

It's a large hotel that has seemed to have trouble keeping its occupancy rate up in the past. I've paid more like £50 - £60 in the past. I wonder whether that £94 rate for most of the month is going to stick. I'd be checking every day or two after booking, in hopes of a drop in the rate.

Thank you!!

WestJet uses Terminal 3. I will look into Premier Inn for T3.

We will check in then go and do something around London and go for a nice but not fussy/fancy meal. Sleeping so close to the airport will be convenient and less expensive.

You can get a standard double at Lancaster Gate Hotel for $172.00 on a Thursday night in June.