I planted a Bramley Apple sapling 2 years ago and it has just (early September in the UK) started to blossom. Is this normal for the UK?
Well it's preferable if this doesn't happen, but it's not at all uncommon in the UK these days. Our weather has always been variable, but seems to be a lot more so now; if you recall, August not only gave us a heatwave, but then turned very cool indeed, especially at night, before turning much warmer in the last week or two. This has resulted in your tree has being fooled into thinking it is spring and it's put out some flowers. This can happen with other plants too; about 4 years or so back, we even had daffodils flowering in December, three weeks before Christmas.
It will likely flower again in spring, at the proper time.