UK companies are charged something called General Betting Duty. For financial spread bets it's 3%. See this document:
https://www.gov.uk/guidance/general-betting-duty-pool-betting-duty-and-remote-gaming-duty
So, one can expect that the bid/ask spread charged on a spread bet would be at least 3% greater than the equivalent CFD to compensate the provider for the duty they have to pay to HMRC.
In fact, they can probably get away with charging even more as the person making the bet would likely be prepared to pay more for having the tax advantage versus the CFD.