we used a Britax Eclipse from mothercare a few years ago. Quite a few have the fitting instrustions to include lap belt fittings, yet everyone simply thinks they will fit better at the side.
We were amazed at the fitting on the lap belt, it felt SO MUCH MORE secure than when it had been fit at the side, after worrying about it it so made sense in the end.
Best you get to a local Mothercare or Car seat shop, the fitters will be able to show you and give advise on the most recent seats available.