Eurostar London to Paris

1) In general, fares go up as more tickets are sold. Buy your ticket as soon as possible to get the lowest price.

2) The trip is about the same duration in both directions; however, the UK is in a different time zone than France (e.g., 9am in London is 10am in France).

3) I find the standard seats fine.

Eurostar fares are staggered. Tickets go on sale at the lowest fare, and once all of those are sold move up to the next fare, and so on until all the tickets are sold out. More expensive fare = more tickets sold on that train.

The UK and France are in different time zones. When a train reaches France an hour is added, while when it reaches the UK an hour is subtracted, hence the different journey times. I believe the train from Lille to Ashford departs at 18.35 and arrives at 18.35!

I like Standard Premier, it’s comfortable and the meal onboard is good, though it can be a big jump in price and if you’re conscious of that Standard is more than fine.

Other advice: get there 45-60 mins before departure for check-in.

Eurostar tickets have a lot in common with Amtrak tickets on the Eastern Corridor of the USA, or air tickets anywhere. The seller wants to maximize income from YOU. So when business travelers are likely to be buying, tickets cost more. After 11AM on a weekday (for example), fares might be lower. Business travelers have less time to plan, so walk-up fares are often several times the price of a ticket bought three months in advance.

Of course, lower fares are usually "restricted". They may be non-changeable and non-cancelable and non-refundable. You may also have to prove you are the name on the ticket, and sometimes even present the credit card used to buy the ticket. (This happens at high-demand Broadway musicals, to reduce scalping and resale of tickets.)

If you can't be sure you will be on time, think about how much it will cost you if you miss the ONE-HOUR EARLY requirement for customs and Immigration formalities - which may take even more time with BREXIT. Are you taking the train upon arrival in the UK? Be careful about scheduling a Eurostar ticket after a flight arrival. Next time, price "Multi-City" air tickets, or "Open-jaw" between the places you actually want to go to.

it appears that the return trip to London is of a shorter duration. Any reason for that, too?

it's 2 hours 20 minutes in both direction. The difference is France is one hour ahead of the UK.

And buy as soon as you are prepared to commit to a date and time. Prices only go up, never down. The more expensive prices you see are the more popular times which are already partially sold out.

And buy direct from Eurostar, not a reseller: https://www.eurostar.com/
You will get a self-print ticket.

Any other input: Unlike other International trains, passport control for both exiting the UK and entering the Schengen Area are done before boarding the train. Plus there is an X-ray bag check. You need to be at the station through the ticket barrier at least 45 minutes before departure, 60 minutes better.
In Paris you just walk off the train.
For more info see this video: https://www.youtube.com/watch?v=l5RAYy9Uymc

The others have covered it generally. As has been said, you'll go through passport control/immigration for France in London, so thats why it pays to be early (again, already been said 45 minutes works well). There is a cafe once you are in the sterile area. Standard seats are fine, but I will occasionally splurge for Standard Premiere as its a bit quieter. Once in Paris, you will literally walk off the train into Gare du Nord.

One thing to note - you can purchase tickets for the Metro in Paris on the train.