Best I have found duty-paid (and I fill up in a lot of places, and have 460-litre tanks) was Teeside, £1.00+VAT.
If they are fuelling a BMIbaby then you might have to wait some time! Have lunch in the St George hotel while waiting - the former Officer's Mess from when it was RAF Middleton St George. Food isn't the best I've had but isn't bad and staff are great (and in at least one case rather charming
).
Before complaining about fuel-company profits, remember that it would be illegal for them to charge anything less. If they sold their crude for less than market price their shareholders would hold them to account, and the people responsible would, I suspect, be charged with fraud or similar. If they sold the fractions (avgas or kerosene) to their distrubutors for less than market rates then they would be fined heavily for illegal cross subsidy. The distributors then charge what they decide is best for business, but as qdmaviation suggests there is not always a lot of room for manoeuvre.