A company simply called "Tracker" (www.tracker.co.uk) makes some of the most comprehensive and proven tracker systems in the UK, but they can be expensive. On the plus side, your insurance premium should be lower, which offsets some of that cost. A good quality, trustworthy tracking system will require a subscription. Even if you choose a system without a subscription it's likely to require a SIM card so it can send information about its location, which is obviously only useful if the device has any reception.

That's why a company like Tracker costs more - its systems use a more difficult to jam VHF radio system to communicate location (on top of GPS), which links directly to police vehicles for more quickly locating stolen cars. Tracker also has dedicated call centres. That said, you could try rewiresecurity.co.uk for a cost effective tracking system. It requires a SIM card, but their small Nano device is only £40 - so worth trying out if you're worried about paying hundreds.