In round numbers a 0.5mm gap in every metre will accomodate rail expansion from a 50C temperature increase, which should be more than enough to cope with temperature variation in the UK. But it is not just thermal effects on the rail you need to think about. How stable is your layout structure with respect to both temperature and humidity? The only reason I have ever found for soldering track joiners is as a work around to obtain a smooth curve when all else has failed. Brian's suggestion of electrically bonding the rails but leaving the rail joiners a sliding fit is the way I would go.