It can be scientifically proved (chaos theory) that, irrespective of any future advance in forecasting technology, it will never be possible to produce really accurate weather forecasts (for a particular area) for more than about 5 days in advance. So any forecast which claims to go further than that is only really looking at general trends and is bound to involve a fair bit of guesswork.
For what it's worth though, uk.weather.com suggests that your area will be starting to warm up a bit in the run up to Christmas:
http://uk.weather.com...day-Plymouth-UKXX0112
Around here in East Anglia there seems to be a greater chance of snow:
http://uk.weather.com...0day-Ipswich-UKXX0072
Geordies might see a few snow flakes:
http://uk.weather.com...le-upon-Tyne-UKXX1695
. . . as might Mancunians:
http://uk.weather.com...y-Manchester-UKXX0092
Snow falls don't look to particularly heavy in Edinburgh either:
http://uk.weather.com...y-Manchester-UKXX0092
Chris