It's quite simple really. Fly at the speed you, or your company, prefer until given speed control by ATC. Then you fly at that speed.
All aircraft can fly at 160 knots to 4 miles, you just have to configure for the speed. The only real reason you should have for refusing an ATC request is to preserve the safety of your aircraft. If you find that your SOP's are too restrictive for Gatwick then get the SOP changed. Remember Gatwick and 160/4 was here long before easyJet and there are hundreds of airlines flying into Gatwick who can do 160/4 without any problem, why should that change?!
Remember SOP stands for Standard Operating Procedure. They can be departed from or even adjusted slightly if required. 'Phone the Chief Pilot if you find yourself continually having problems with an SOP and he can and will change it if he thinks it's appropriate...and in the 160/4 case it would appear to be so.