Firstly, to the best of my knowledge any airline will only load sufficient meals to meet the orders of those booked and paid for. So no, you will not be able to pay for them on board.
Secondly, last year TCX acknowledged that people were having a problem getting through to them by introducing a call back service. They will normally respond within 24 hours and of course, they will now be paying for the call. To access it:
Go to
Fly Thomas Cook | Cheap Flights - Offers & Deals Online
In the bottom right hand corner of the page click on 'Help'
Under 'Pick a topic' click on 'How to contact us'
In the dialogue box on the right click on the words 'Click here' in the second paragraph
This will bring up a screen for you to enter your contact details.
Personally I think that they are doing you a favour.
Have you ever had a meal on TCX? Not exactly in my Top 10 of best airline meals. It is only a four 1/2 hour flight after all and I would have a hot meal at the airport and then take a sandwich or other snack on board.