On these flights there are BOB products which cannot be given away free by the crew as they are owned by a company called Tourvest, other than the products loaded in the front galley there is very little in the rear galley for ET customers.
From LHR there are small bottles of water, pretzels or breakfast bars and all of these are handed out after takeoff, from LGW there have large bottles of water which they use to pour into glasses plus the same pretzels and breakfast bars.
In the OPs situation on LGW flights they could only give out more water if they had plastic glasses left and would they would have to use the water from the aircraft water tank, for LHR flights they only have about 30 plastic glasses for use with the sold BOB products.