have you phoned to see if they have any on-site disabled parking facilities. Lots of the big london buildings have a copule of bays that you can pre-book. Some of the cabs are high roof/low floor accessible cabs not sure if you cna pre-book or if you will just need to take pot luck.
There are some spaces near the London Eys so you can just walk accross the bridge, there are also a couple of car parks there, the one under the festival hall is OK.
Or have a look at Pimlico parking, there are a few car parks there.
If you're looking for a cost effective way - there are some tube stations that are accessible for wheelchairs. TFL's website has a list of those that are - and most connect with mainline stations. Does depend on the time of day though. Main commuter time is hellish full stop.
I had a look at the tfl webpage and its 2 buses, but to be quite honest, I don't think she could cope with 90mins train followed by 2 buses THEN be on best behaviour for a reception then the same all over again to get home!
I like the funsizemars idea!! I have 3 minis - shall I call them minimars, midimars and maximarsbars?
usually when I have to talk about them in an abstract way (sounds bizarre put like that! we have had huge issues with dd2's name, and the passport office in their wisdom ahve re-named her, so we ares till fighting to get her actual name on her passport. I've had a couple of threads about it, and for clarity have then used silvertadpole as one of her names!)
maybe your dc could be treat size, fun size and king size