Received a google play gift card as a gift but I get the feeling these are not much use for an iPad.

I have downloaded a google play books app so perhaps it's only book.

It was purchased from tescos and I know they don't take branded gift cards back

they're mostly of no use if you have an iPad. the Google Play Music store will let you download mp3s though which will work. And, you've the option of buying the books as you've seen.

Apps won't work, and I don't think the movies will either....

Yeah, music is probably your best bet - you can download them as drm free mp3s and import them into iTunes, and Google Music is actually pretty cheap in comparison.

Yep,

if you buy music, the only big difference is in price. Only laziness/convenience, and obviously gift vouchers, stop us shopping around.