Food/Cooking-related must-do things in the UK?

Ottolenghi --but I bet you already thought of that...

River cottage has quite a range of interesting activities, I think, but it's in Devon, so I don't know if it would be worth the extra travel for you. http://www.rivercottage.net/

If you want to have some really good Dim Sum (yes, I know, not really what you'd go to London for), I would recommend Pearl Liang. It's a quite hidden, just behind Paddington Station, and really, really good.
And definitely Ottolenghi, and their new restaurant NOPI.
Moro might be something else I'd recommend, but I haven't been there myself (I just like their cookbook).

Hopefully I will be returning after this trip (if my son gets into Cambridge for college). River Cottage has some really interesting looking classes! And actually, I had NOT thought about Ottolenghi, but will definitely make that happen. So is NOPI their best restaurant? Each one is different? Oh - my poor diet and exercise regimen!

Dim sum -- we love dim sum so will try to find this place if we are around Paddington Station. I don't have my bearings yet -- all I know is that I will definitely find the Tate Modern, and I'm glad I am spending a few extra bucks to stay downtown.

Lunched today at NOPI. Amazing!