Probably Hedera colchica varieties - will be 33 feet after 5 years. Bear in mind that anything that grows rapidly will also get rapidly out of hand and may end up much bigger than you ever needed.

The fastest growing ivies must be all the ones which invade from my neighbour's garden and I'm forever cutting back... as Bamboo says.... they get rapidly out of hand....and can invade other gardens nearby....:o(

Algerian Ivy (H. canariensis) is slightly faster than H. colchica, but I don't know how well it would take the the Dorset winters.

Of the Hedera helix types, there is the form Hibernica with larger leaves. This is also quick (relatively).

Interesting, 'Hibernica' looks exactly like the form we sell in the States as 'Baltica'.