LHR's fees are slightly higher than other UK airports, but the real issue is UK APD (departure tax) which will be charged for stopovers >24 hours. Transit (<24 hours) does not attract this fee. You'll be hit by this whichever UK (mainland) airport you use. In the general scheme of how much the redemption will cost in YQ, the other differences are probably not worth worrying about.

I thought miles trading had been outlawed on Coupon Connection because of the legal position, but correct me if my mind is playing tricks and it was something entirely different.