well you havent entered a legal contract with the agency, so lying to them wont harm your new role (although it they find out you may find yourself blacklisted by them).
Frankly its a bit of a gray area but an offer of employment isnt made according to your last salary, so lying about wont haunt you until and unless they ask for your P45 etc - in which case you could be dismissed not for lying about your salary (the contract wont have a clause saying you are employed on condition you told us the truth about your previous salary) but there is an implied covenant of trust in any employment contract and lying about previous salary could be construed to breach this. It is less serious than lying about dates of employment or qualifications/grades however.
Not sure what you hope to gain from lying to the agency though - if you dont trust them to represent you properly and get a credible offer at negotiation stage, dont let them. If you DO, why start a relationship with them by lying?