It looks fine to me if everything goes reasonably smoothly.
My calculations would run along - leave 90 minutes to walk through the frosted glass doors at Heathrow (might be less, not terrifically likely to be much more than 30 minutes more unless it is right when regulations change, by the end of July is probably OK) so that's noon.
It doesn't take half an hour to walk to the Tube unless you each use the toilet for a long time sequentially, so round down and call it 70 minutes including getting an Oyster or using a contactless card or phone each to get to St Pancras Kings Cross. That's 10 past 1. You need to be in the queue to check in for the Eurostar at 1:50. So you have 40 minutes to scrounge food from the very many choices at St Pancras and across the street at Kings Cross. Then again you could blow most of that time deciding on which combinations of food. Maybe check the website for the stations and do all the drooling before you travel so you don't have to decide in a jet lagged fog.
If you run out of time before checkin there is a Cafe Nero in the waiting room.
Word to the wise - I found out the hard way that the security (first step immediately after check-in) make you throw away cups with coffee or tea in - they don't want loose hot liquids in their machines. Another check in the box for the after check in Cafe Nero.
Would I do it in your shoes with the temptation of all that money? dunno.
It is a good thing that July is a way off. I just checked for my trip by car to Belgium and found that I'd have to take tests and quarantine until the result to go to Belgium and stay more than 48 hours - even fully vaccinated. I will make sure that I only transit Belgium if they don't relax that rule.