When is celery in season in UK?

Answer 1

Vegetables no longer have 'seasons' in the UK - since most of them are imported, or grown in hot-houses - they're available all year round.

Another answer:

The UK imports about 60% of its vegetables and 90% of its fruit which means that 40% (nearly half) of UK vegetables are still grown in the United Kingdom and are all harvested during the appropriate season. A large amount of celery is grown in the UK and it is normally harvested in July/August until the first frost in Oct/Nov.