The bit you need is ANO Section 3 General Regulations which lays down the performance requirements for all of the pre-JAA categories of aircraft. Piston twins are normally Perf C (if they'll probably climb away on 1 engine) or Perf E (if the live engine may only take you to the scene of the crash).
The ANO link is:
http://www.hmso.gov.uk/si/si2000/20001562.htm
Use the performance tables or graphs in accordance with the procedures in the ANO. Be careful about public transport factors on Perf E aircraft which are probably not included in the graphs. Perf C graphs normally have the factors (different ones) included.
The perf group of the twin you're flying will be stated in the UK supplement at the back of the POH, but here are a few I know:
Perf E
Seneca I and III
Duchess
Seminole
Cougar
Partenavia
Perf C
Seneca II