Whoa - be really careful here.
Escrow is great advice, but there are a significant number of current scams operating trying (and unfortunately often succeeding) in getting people to pay deposits to fake escrow services, and losing hundreds or thousands of pounds !!
Since 1 November 2009, with the introduction in the UK of the Payment Services Regulations, escrow services finally come under the auspices of the FSA and HMRC.
You will find genuine escrow companies at the FSA online register at www dot fsa.gov.uk/register/psdFirmSearchForm.do (try searching there for 'escrow' as an example) and/or at customs.hmrc.gov.uk/msbregister/checkTerms.do (try searching there for 'tru' in the name box and 'NW11' in the postcode box for one such genuine example).
These new online escrow services may offer exactly the service you are looking for, and cost only £3 or so all-in to use.
Beware, there are many, many fraudulent escrow services which look real - even quoting UK addresses, phone numbers and fake FSA & HMRC registration numbers. They look extremely professional, and any money posted to them (or goods sent relying on them) will be lost forever. So only ever work with a firm on the FSA register and/or the HMRC register, which will be safe.