Rome brought stability to Celtic England Pax Romana. Many Celtic leaders took on the trappings of Roman Society. An example is Fishbourne Roman Palace built for/by the local Celts. Romans are known to have imported domesticated apples into Britain. There is a reference of obtaining 'good' apples on Hadrian's wall. Vindolanda wooden writing tablet 302
"… bruised beans, two modii, chickens, twenty, a hundred apples, if you can find nice ones, a hundred or two hundred eggs, if they are for sale there at a fair price. … 8 sextarii of fish-sauce … a modius of olives …"
It is not a giant leap from Romans having domesticated apples to making cider knowing that Celts made a prototype cider from native 'crab' apples.