Thanks for all the advice, sorry for a late response but it totally forgot, as that was also a busy period for me.
In the end we opted for:
Roast (Borough market) – this was actually very expensive, but good food.
Launceston Place (Kensington) – very good.
Harrods – for afternoon tea, which was superb and good value!
Brass Rail (Selfridges) – one of my favourites. It’s a Salt Beef Bar and a real gem.
We were unable to get in at some of the other places suggested due to availability, but do have them on my list for future events.