Britain was isolated from the continent. The Celts did control the secret of iron for a long time, but Britain wasn't some kind of distant albion. I can imagine that the celts would have found a lot of sport on that land and hired themselves out to the highest bidder. And aren't the Myrcharn Belgae horsemen, not british horsemen, anyway.

Besides we cannot control when mercenaries come into existence in EBI, the RTW engine does not allow it.

Perhaps some evidence for your assertion that Britain was isolated and that its military was predominantly less developed would help. We do know from the Vindolanda letters that the northern tribes (in caledonia) fought bareback on horses without weapons, but the southern tribes were certainly more heavily influenced by the developments from mainland gaul.

Foot