Westminster on Day of Arrival?

Westminster Abbey, right? You shouldn't have a problem with nearly four hours to spare. You'll have the passport formalities before leaving Paris, so you just get off the train and taxi or tube to your hotel, drop your stuff and get over to the Abbey.

Yes! Westminster Abbey!

It’s doable in theory but to me it’s an expensive admission fee and you’re depending on the train running on schedule. Last time I traveled Eurostar (April 2023), it was exactly on time. One of the previous times it was 101 minutes late.

It’s up to you but are you working with a short time frame for London? Is there an alternative sight you want to see that does not require a timed entry?

There is room for debate on whether you NEED to pre book for the Abbey.
The evidence is not, in the afternoon at least.
It looks like you can turn up. If you wanted to, when you are through the tunnel so on English soil with a better idea of timekeeping, you could buy then, if that makes you more comfortable than just turning up.

Best to check if Westminster Abbey will be available to visit. Often closes for events.

https://www.westminster-abbey.org/

Recent industry reports show Eurostar delays are under 10 minutes across the day. You can get to your hotel from St. Pancras fairly quickly, under half an hour. I would be booking the Abbey much earlier than 1430.

If you have any issues on the day they are very amenable to helping out

In looking at this Monday the 19th, there are still 225 tix left for 14:30 and 250 for 15:00. I think the advice about not pre-buying and just buying same day tickets when you arrive is probably fine. As others have said, that is an expensive ticket to waste if you end up delayed.

Thanks for the advice! We only have a couple of days in London so we are trying to make the
most of it! We will look at buying tickets that day.