Time is going to be the biggest thing you'll save. If you want to save hard cash then 'IF' you complete your purchases every month, you can then submit your Self-Assessment Online yourself rather than employing an accountant. Check out the walkthrough guide in help.
The Self-Assessment rules HMRC produce and revise annually, which guide you through what you can claim for against earnings, are built right into OrderBook. The guide of what can be claimed for, under different headings, are where you need them. It's not as difficult as you might think. Check the links page for links to the HMRC Self-Assessment web site.