There is some evidence that the Phoenicians / Carthaginians got as far as Ireland. Ireland managed to stay independent from Roman Empire, when Rome collapsed they were powerful enough to launch some raids on post-Roman Britian.
Even if the Romans take control of a Phonecian or Greek colony in the British Isles its possible this entity could survive imperial Rome. The eastern empire remained 'Greek' after several centuries of Roman rule. A Greek Cornwall, or S Ireland could remain Greek post 500 AD, at least until diluted by assorted other Frisian, Saxon, Norse, & Norman invasions