Really, the only way is to get a tracker installed. Given you're in the UK, if it's Thatcham rated, you can probably offset a large chunk of the cost of it against the insurance reduction.

Most of them now have multi-mode tracking (i.e. can override GSM jammers if stolen), and will allow you to track via smartphone in situations other than when its been stolen (the old ones used to just be on "alert the police" mode or nothing).

Alternatively, just get the sticker saying there's one installed