Depends what you want to do.
If IT consulting is your bag - then Detica work on client sites in London and Cheltenham building bespoke systems. Also at Cheltenham are IBM and PA. Atkins do some Project Management work there, or at least they used to.
If you actually want to do some intelligence analysis - try Janes Strategic Advisory Services, but the pay's not great.
Be warned though, all employers will expect you to already possess the security clearances you need, getting them from scratch as a civilian contractor takes at least 6 months, during which time your employer will not be able to utilise you on the projects.