I’m Uk and VAT isn’t an issue for two reasons. Firstly you don’t *need* to be VAT registered until turnover is over £50k (or somewhere around that) although you can register if you want to claim VAT back on your outgoings, and secondly you don’t have to charge VAT to overseas companies, and all the casinos pay from overseas companies generally. That may change in the future once regulation is in place.
I choose not to be VAT registered as with my last company it was a PIA doing those things every quarter – you’re just a glorified tax collector – but if your outgoings with VAT are substantial you may feel it’s worth it irrespective of turnover.
Invoices, protocol would dictate that you should keep invoices, but as long as you keep some sort of record of payments you should be ok. I certainly don’t send invoices but I do keep a record of all monies received.