Not only should be asking about travel compensation, you should ask about being paid.
Start by reading up on this page, which talks about compensation as an intern.
Your school work experience was not paid, and that is normal. The point is that you are learning about the workplace, so being there is part of your studies. Work experience is specifically exempted from minimum wage and other labour laws.
However what you are about to do is useful work for the company. My reading of the page I linked to is that you are legally entitled to at least minimum wage, unless the game developer is a charity, or unless you are still under 16 (I am not a lawyer of course). The going rate, even for a school student tester, is probably higher than that. It's worth pointing out that even if you have agreed to work for free, minimum wage laws still apply - in other words, nobody can sign away their right to be paid minimum wage, and an employer who didn't pay that would be breaking the law.
Even if for some reason they really don't want to pay you (perhaps by trying to claim that what you are doing is not really work), and you would rather work for free than lose the opportunity, at least ask for a travel allowance. £40 per week is probably less than their coffee budget.
If you never talked about pay before, here is how to do it.
- Talk to an adult who knows something about the world of work, such as a teacher or relative.
- Find someone of similar age doing as similar a job as you can and find out how much they are paid.
- Contact the company. If you have literally not talked about pay with them, just ask how much they were planning on paying you. If they have previously indicated that this would be unpaid, or if that is what they say now, tell them you are excited to be working for them, but you need some sort of wages to cover expenses. Also you believe they are supposed to pay you at least minimum wage.
- Have a figure in mind based on the research you have done. If they offer less than that, suggest that figure.
There is of course much, much more to negotiating salary. Do more research if you are motivated.