I operate in the UK and would look at your question from a different perspective. I wouldn't alter any of my accounting. Instead of selling an item with a price in bitcoins, keep it in GBP until the point of payment and then treat bitcoin as a payment mechanism like any other like a debit card, voucher, cheque etc. I'm assuming in the example below you are VAT registered.

You have a laptop to sell to someone in the UK worth £200 + VAT = £240. They also need a power cable at £5 + VAT = £6.

Total Net Value - £205
Total VAT - £41
Total - £246

Assume GBP/BTC rate is 1 BTC = £24.60. You accept 10 BTC to settle the invoice, transfer it to GBP and continue performing your accounting in GBP.

Hope that is of some help