Do you have any fairly standard wheat ales under your belt? Everyone could get behind a fairly stock standard wheat ale. But being December a warmer maltier beer would be fairly appropriate as well.
The pilsner and the IPA will probably be drank up the quickest. I would bet that the trappist I would personally go for a lower abv version, A) to save a bit of money cause they can get expensive fast (even if money is no object), B) if you start serving a 9% abv beer the beer drinkers will drink it like beer, instead of like wine and the party could get super rowdy super fast. You can also make more of a 6.5% tripel or dubbel to make sure everyones glasses stay full.
For the IPA were you going for a more dry crisp IPA or giving it a good malty back ground? I would take this into consideration. Not to say a good pilsner doesnt have good body and mouth feel, but if you have an IPA (which will have a dry mouth feel just because of the hops), then a pilsner which most people will perceive as generally dry, then a trappist style with high carbonation. I would personally go 1 of 3 routes.
A simple straight foward fruited beer, raspberry wheat for example. Something simple, light, low abv, fruity as all get out (insert your favorite fruit here). This is 100% not intended to be sexist as I LOVE to make fruited beers and raspberry wheats I will always reach for, but the ladies do really love a good raspberry wheat.
Another route could be a simple, malty, english style ale. English brown ale, little roasty, little malty, little bit of EKG/Fuggle aroma. 4-5% abv, no one can dislike a clean malty low abv english ale.
The last route I would consider taking is due to the time of year. Being middle of winter, I don't know how cold your winters get in London, but a milk or oatmeal stout would probably be super popular. I don't know if I've ever met someone that dislikes oatmeal or milk stouts (maybe 1, but not the other).
The other things I would take into consideration is what you are serving for food at the reception. Throwing something seasonally relevant is a nice touch and you can put on peoples cards at their tables "The IPA on tap will match well with blah blah blah meal" will add a personal touch to the meal. If you are not confident outside of your 2-3 recipes you have down, I would go the route of checking out other peoples recipes and try to replicate those depending on what you decide for a 4th offering.
You could go the crazy route as well and whip up a simple batch of cider as well, lots of recipes/guides on here with good instructions for making a really simple cheap and fast cider. If I went to a wedding and they had 3 homebrews and a homebrewed cider I would be completely blown away, in fact I think I may add this to my ideas for potential brews at my own wedding reception.
