ry going along to your local GC and see if they have any this will give you and idea of the size of the grown onion. BUT later does not necessarily mean better! It is the flavour you want not a big flavourless variety that will do nothing for the meal when cooked. Of course if your growing for showing you'll need to start of with onion seed not sets.

'Stuttgart' is a commonly available kind that can get fairly large. If you are trying to win a giant onion contest, those varieties are generally found as seed, rather than sets.
All kinds will size up better with good culture:
Soft, rich, sandy, well drained soil with ample supplies of nitrogen and potash.
Mycorrhizae present, especially Glomus species.
Generous spacing of individual plants, around 20 cm apart in the row.
Ample sunshine--don't plant under trees, or between buildings.
Keeping the soil moist while they are growing--probably not a big problem in the UK, but the hosepipe may occasionally be necessary.
Keeping them in leaf as long as possible before they bulb up, including planting in the fall, if possible.
Best of luck, Alfie!