www2.wetter3.de/ is my primary site, I look for weather fronts and wind speed, since fronts are the principal cause of significant weather and are to be avoided, and windspeed dictates if it's within our safe limits to fly. This site is better than the met office site as you get an extra couple of days forecast.
Click on the down arrow under Bodenanalyse, this steps through the next 5 days and you can watch the fronts moving and see where and when the high pressure will creep in. This is very useful to know as you can plan when/if a front will pass over which is often not obvious on one chart alone.
Here you see (in 2008) Wednesdays forecast at midnight, a lovely big lump of high pressure covering the UK, the lack of isobars (lines) shows low winds, high winds would be indicated by the isobars being close together for example at the top of the map. Also the lack of fronts crossing the UK means no significant changes in the weather (usually rain and temperature changes). The next map at 12:00 is virtually the same (not shown here as it just clutters up this section) so we can deduct there will be little change during the course of the day. By stepping through the 5 day forcast you should be able to tell which days are looking flyable and how long the window willl last, and this day looks peachy.
Next I like to confirm the weather in the location I want to fly, I know the met office has spot wind charts however I swear by www.metcheck.com/ which translates the surface wind into a very easy to read format. Simply enter your location in the box and step through the days of the week.
To confirm what we thought about Wednesdays weather, Metcheck shows the following detailed information throughout the course of the day including cloud cover, windspeed & direction.
Metcheck does have an aviation forecast but I find the basic one does the job. I play with these sites a lot, and while there are more sites to look at, these two simplify the forecast so I'd advise you to look at them as often as possible, clicking through the days ahead and see how accurate they are. The weather is like an episode of Eastenders, you can pause one frame to see who's in it, but you really need to watch the episode to see what they are doing.
http://www.XCweather.co.uk is a superb realtime windspeed website, use this to check the difference between forecast windspeed and actual windspeed as this gives a very good indication of whether or not it will be blown out.
Then there is the Met office balloon forecast's for morning & evening, always worth a look before you leave for a fly to confirm that it will be a nice one !
Remember a forecast is a predicted event, but it has variables, however as the event draws closer the variables reduce and the forecast becomes more certain until the event actually happens, so don't rely on what it says will happen in 5 days, check it each day.