VAT is payed by the person purchasing a product and basically you just act as the middleman passing this on, on behalf of the tax man.
Casino players do not pay VAT when they make their deposits, therefore how can you give the tax man your portion of the VAT that corresponds?
I think you would only be able to claim VAT from casinos based in the UK if that, as any which are licensed elsewhere do not pay VAT or taxes in the UK. And even if that was true then you would only have to claim VAT for your UK players….
Just my 2 cents.