If you are making more than 64k in taxable supplies in a year then you have to be VAT registered in the UK.
That means if you are invoicing UK companies which are VAT registered you need to add an additional 17.5% to your invoice. This 17.5% is then paid to the nice VAT man.
Companies registered offshore you dont add VAT.
And dont forget to register if you are eligible..there are penalties for late registration..I know! :sarcasm:
In fact I had to invoice many casinos/UK bookmakers just for the VAT which I should have charged in the first place. Now THAT was a PITA!