Pavlova is always a hit here - I use Nigella's recipe, and serve with lashings of cream and fresh raspberries. If you have it, use raspberry vinegar in the pav - it adds an extra layer of deliciousness.
Or there is my go-to pudding - a sort-of lime cheesecake but with no cheese.
Blitz ginger biscuits and add softened butter, to make a biscuit base. Press into flan tin - I put it up the sides too, it is easier to get out that way.
In a bowl, mix one big tin of condensed milk and the same amount of double or whipping cream, add the zest and juice of about 3 limes (or two lemons, if you prefer lemon) - the citrus juice will thicken the cream and condensed milk mixture, and you can pour it into the biscuit base, and refridgerate to set.
You can taste it as you are adding the juice, to get the level of flavour you want (and it does depend on your limes/lemons - some are juicier than others, so you might need a bit more or a bit less juice than I have said).
Take out of flan tin, cut and serve with cream.
Another thing that was a hit here was Delia's mini sticky toffee puddings - and you can do those ahead of time and just heat them up on the day, I think.