Some sportsbooks recently licensed by the UK Gambling Commission and now online for UK-based customers to use:
www.betonbrazil.com (promoted by Alan Brazil)
www.energybet.com (official betting partner of Aston Villa)
www.grosvenorcasinos.com/sport (powered by Kambi)
www.beteast.co.uk (powered by TGP).
I haven't tried any of them myself yet, so can't recommend or not recommend them.