My experiences at the hotel were:
* as a plat, I have been upgraded to a nice room on each stay. It's not the best hotel in terms of room quality, stunning x or y, etc. It's a nice room.
* The hotel is old and there are some creaky floorboards here and there. I don't care about that too much, but others seem to care.
* the staff was nice to me on both stays (as plat). I'll suggest that sometimes in european properties the staff may not be quite like an american property, meaning that someone from a european country checks you in. there can be a few cultural differences, but I don't think that I've ever been treated "poorly" by anyone there.
* I liked their buffet breakfast (which I expensed, so the that's more of a comment on the food than the food/money value)

I also stayed at Le Meredien Picadilly in April and I posted my comments in that hotel's thread. It has been updated more recently, has a club lounge for plats, etc. It is a better, newer hotel. The suites there are nicer.

It depends on what you want - my experiences with hotels are very different now that I have status in the SPG program, and I think you should qualify your expectations both personally and for what feedback you get. The experience of a plat at one of these hotels is different and that's a good thing - that doesn't mean that the non-plat experience is nice, but i think you'll see that the frequent guest has high standards and low tolerence for annoyances.

I hope you have a nice time in London!