You could use Excel, depending on how may transactions and how complicated your business is. You do say that it'll be less than the VAT threshold.
If you have plans to expand in the future, or you see yourself exceeding the threshold, you should seriously consider some accounts software. It makes producing balance sheets and P&Ls a great deal easier. You will probably need to do some analysis (such as cash flow) to make sure you're not going bust.
SortMyBooks or Mamut (I'm affliated with Mamut), both of which offer free trials.
Do you also want to process pay for your employees? You can try Payback (Which I'm also affliated with) [broken link removed]