Need more info , so here's lots of questions:
1. How old is the tree (or how long have you had it)
2. Is it the only apple tree either in your garden, or anyone else's nearby?
3. Where in the UK do you live (for climactic reasons)
4. How many years has it produced only one apple for?

And, how big an apple? If it's a two-ton monster, then that's the answer! As Bamboo says, some questions to pursue before a definitive answer is given. Compatible pollinators and location are prime factors. Worthy

More questions:
What happens in the spring when it blooms? Does it just produce a few flowers? Do you normally have a hard freeze during or right after bloom? Do you have a good population of bees while it is blooming? Do you prune it hard every winter? Do you feed it during or right after bloom?
All of these factors can drastically affect fruit formation.