I can't advise specifics for the UK, but in general the tax people are going to be more impressed by seing the money moving in and out of your bank accounts than they are a piece of paper.
If you have a record of the money going out, any sort of receipt and some sort of packing list or even customs paperwork on the goods, you should be covered.
In fact, more important than an Invoice, which is saying "You Owe", would be the PayPal Recipt, which is saying "We have received your payment."
Most tax people are looking at formulas, relationships, etc. If your cost of goods and your resulting income fall into line for your industry, they should not get too interested in line item detail.
Much more questionable would be claimed expenses and things that would cause your Net Income to be out of range with your Gross Receipts.
I'd be surprised if there is anyone on this forum who can say they have been involved in an audit that got down to line item questioning of invoices. A review of your bank account can often tell them all they need to know.