There's a bit of a list:
Lack of either potash or magnesium.
Too much salt (sodium) in the soil or water. Probably rare in the UK, unless you have a water softener.
Some pests, especially thrips or aphids.
Virus disease--hopefully not, as that's incurable.
A mutation, or "bud sport".
Too much cold.
If just one or two flowers, it may just be the plant making a "mistake" as it grows.
I hope this helps with the hunt!