I think your caterpillar is that of an Elephant Hawk-moth....

Text from NHM website - Hawk-moths are some of the largest moths found in the UK so its stands to reason that their larvae, or caterpillars, are as equally impressive. The caterpillar of the Elephant Hawk-moth reaches a very impressive 7.5 cm in length and is a tan in colour, with a ‘horn’ or ‘spine’ on its tail. The head appears to protrude from the body and can be retracted and is suggested to look trunk-like, hence the common name of this moth. When threatened the head is retracted which swells the body displaying false eyes on the body. It also thrashes from side to side and it is suggested that it is mimicking a snake. The caterpillars feed on willowherbs and fuchsia plants in gardens and despite the exotic appearance of the adult moth it is a very common garden species.