Apples are one of the easiest fruits to grow in the UK, with a wealth of different varieties and uses for them although, that said, if you haven’t grown them before and are starting from scratch there are a few things to be aware of.
Root stock – most apple trees are grafted onto a rootstock that will keep them smaller, and the rootstock numbers are pretty random.
Pollination – there are a few self fertile apples but even they do better if there is a pollinator about. Crabapples are universal pollinators so if there is one of those locally then your tree will be fine.
Flowering time is also critical to pollination and so apples have been grouped into numbered groups – the RHS list can been seen here.
And the last thing to be aware of is that some apples are triploid. This makes them incompatible pollinators as they are sterile, having 3 sets of chromosomes so 3 plants with 2 being haploid varieties are required.