Billhooks were used in Britain before the Roman Era (2nd invasion c 50AD) - iron tools were made and used in the Glastonbury Lake Villages - much of what is now the county of Somerset was at, or below sea level, and the region consisted of a series of lakes, swamps and marshes. Whether they were developed locally, or the idea was brought to the country by traders from Europe is not known (but trade with Europe was well established by the Iron Age, and had been so since the Bronze Age). Phonecian traders, buying Cornish Tin to make bronze, may have brought tools and weapons to trade.
Certainly during the Roman occupation, up to about 400 AD, the billhooks became widespread throughout Britain, and also spread from the Mediterranean to Northern Europe.... Many example survive from this period, and already a variety of blade shapes and sizes is evident...