Airport prices in the UK for recreational travel work like so:

Flight from London to Barcelona: £16

Bottle of water past security: £5

Train to airport: £26

Taxi enters drop-off area for 30 seconds: £7

A person who wants to get the advertised flight at the advertised price has to be very careful.

When people say "water" here I have to assume they mean "vodka". Otherwise you can just bring an empty bottle and fill it on the other side. It's the toiletries that pose a problem.

Some airports charge money for water after security.

Others disallow even empty bottles at security screening