I don't agree. The prices onboard are fairly reasonable. At our local airport it's £29 for a bottle of prosecco or £32 onboard for a full bottle of champagne.
Get a Burger King it's around £8 or a WHSmith meal deal is £6 ish. A hot meal, drink and snack around £6 onboard. £9 if you want it with a beer or wine.
I flew domestic and bought breakfast for four of us with drinks and snacks for around £20. Not bad.
A double gin and tonic £9 in the airport. £6.50 onboard. I've paid £10 for a double g and t in my small northern town where bar prices are a lot more sensible than many places.
I can't think of any airlines on short haul euro flights who do stop you bringing your own onboard so I can't envisage BA doing it.
Any Spanish airport prices are obscene for a bottle of water. I won't pay it. I'd rather wait and get one onboard. I saw a white chocolate kit Kat at AGP before they came to the UK. Oh how excited I was, until i saw the price tag of €2.50. Id rather starve.