Agree, definitely make sure the operator has an AOC (Air Operators Certificate). Helicopter Services at White Waltham would be my recommendation (though they are based at the 'wrong end' if you need a same day return journey). However their Robinson R44 (single engine, piston) should still be quite a bit cheaper than the £6,500 + VAT you have already been quoted; I suspect their AS355 (twin engine, turbine) may be a bit less too, though my pricing may be a bit out of date.
Alternatively, I believe SaxonAir Helicopters at Norwich have an AS350 (single engine, turbine) on their AOC, though it wasn't listed on their website when I last checked. However they may be worth a call, as they are a bit closer to the pick-up point and should be more cost effective than the price you already have, assuming it's a same day return.
Helicopter travel can be great, but do bear in mind that weather can prevent flights, especially if you opt for a single engine helicopter, so you need to have a back-up plan to drive and be prepared to use it!
h14