Chartered Accountant here. You registered as self employed with HMRC last year, so you will need to prepare your first set of accounts from the date when you first starred to freelance until 5 April 2019. Although the filing deadline is not until 31 January 2020, I suggest that you submit your first tax return sooner rather than later, whilst everything is fresh in your mind.
I agree that CGT is not an issue for you. You should not include your UTR on invoices. People working as sub-contractors in the construction industry put this information on their sales invoices, but it doesn't sound to me as though you are within that category.
A sole trader is undoubtedly running a business and is self employed. All of the invoices raised through the freelancing site need to be included in your tax return. If they have charged you for this service, their costs will be an expense which can be deducted from your income.
You can claim expenses relating to your overheads - bank charges, mobile phone bills, ink for your printer, stationery, travelling expenses, relevant professional subscriptions, software etc.
I use QuickBooks for a few clients, but I don't bother for my own accounts and just use a spreadsheet. I really wouldn't bother incurring the expenses. Consider taking out professional indemnity insurance depending on your line of work. You don't want to be sued for negligence by US clients.
Kyles has offered some sensible advice, but please ignore the rest about self employment/sole trade/ running a business from home. A little knowledge is a very dangerous thing!