Sounds like a lovely trip, the barge sounds great. We did that for a few days around a bit of England a few years ago, it was fun and a real experience. I'd love to do it in France as well.
If you are around Borough Market a nice area to go is Bermondsey Street, it's about a ten minute walk from the market and has lots of pubs, cafes and restaurants. http://goo.gl/maps/BtXen. It is also near Tower Bridge. There is a nice Spanish restaurant there called Pizarro. http://www.josepizarro.com/restaurants/pizarro/
Also if you're staying around St Pancras then Bloomsbury is a nice 'old London' area to walk around, Lamb's Conduit Street has pubs and shops and is nice to walk around. http://goo.gl/maps/tNBjJ
It's hard to recommend a pub, there are so many of them that are nice and old fashioned, you will pass many when you're exploring so just pop into a few!! I'm sure you'll be able to tell the nice ones from the chain ones just by the outside. The best pubs to eat at tend to be local neighbourhood ones outside of zone 1, but if you are only here for a couple of days you probably won't have time to get out of the centre. The pubs in town are better for a pint rather than a meal I'd say.
If you want somewhere special to go for a meal there is a place in Soho called Bob Bob Ricard, it's pretty unique, the food is great and it's a lot of fun. We go there a lot and have also taken my husband's parents there, so it is somewhere that I'm sure your in laws would enjoy as well. I'm sure you could walk in for lunch on a weekday, evenings you might want to call ahead. http://bobbobricard.com
Afternoon tea is a lovely thing to do. And getting the Eurostar is such an experience, I love it. No matter how many times you've done it you are still amazed that you get on the train in London and two hours later you are in Paris!! It's crazy. St Pancras is a beautiful station, while you're there you could check out The Gilbert Scott bar, it is quite grand and old fashioned.
Let me know if you have any more questions!
FANTASTIC!
Thanks thanks SO much for taking the time to do all of this for us -



I will check out all of your recommendations and yes, walking around Bloomsbury will be lovely.
We're getting pretty excited about this adventure. Love traveling by train - and this will be my first Eurostar experience crossing the channel!