I actually got a fairly decent and quick reply, though the gist of which was that the slogn was because it was only men/boys tshirts and the lack fo female game tshirts was probably one of two things.
1) they didn't think they'd sell
2) the stockists don't make them
Regardless of which is the case I got told that they'd pass on my request to the buying team and that any other female gamers who wanted there to be more gaming tshirts could similarly contact Tesco's to say they would like to see them, as if there was enough demand proven then they might starting looking to stock them.
So if you're a female gamer in the UK, or know of one who might be interested in this, and you want more gaming tshirts at Tescos the email to politely poke them about this matter is: customer.service AT tesco.co.uk
Comments
Remember that the back of the shirt needs to be higher, so cut that a bit more shallowly (I usually go just along the back of the collar, at a wide angle than the elasticated collar itself). I'd also recommend putting a stitch at the tops of the shoulder seams, just to stop them unravelling.
There's another method here, which is basically cut off the sleeves and use strips from the arms to rouche the sides, but it's a bit odd-looking.
Another alternative is to just cut off the sleeves along the seams, do the collar as I have here, and then use excess strips of material to tie around the tops of what used to be the shoulders to make them narrower with little bows on top.
I'd suggest using an old t-shirt you don't mind wrecking to try it out on, first. :)