you need to use an approved method of converting the US$ back into GBP. Typically, a month end rate will be used by most companies, in the extreme a daily conversion but this is unusual from what I've seen in practice. They would generally take the financial times rate on that month end date.
Your revenue in US$ x FT exchange rate = revenue in GBP
after conversion, you then deduct any expenditure to arrive at your taxable profits. The FT month end rate is a well established rate used by HMRC.
check with them, they probably have loads of approved conversion methods if the one I've mentioned doesn't suit you