A 5-hour should be possible at Portmoak during all but the depths of winter - *if* the weather co-operates, and can be a big 'if'. A silver height is possible at any time of the year if you get wave and can get into it.
However, silver distance for a visiting pilot without their own glider is probably a lot more problematic because of the issues of landing out a club glider when you are a visitor. I suspect that as a visitor you would not get permission to flying a club glider other than locally.
Do you have the XC endorsement? If not you will also need that before attempting the 50k at any club.
You might do better to use the winter to make sure you are current flying gliders (both winch and aerotow) and have the XC endorsement ready for April next year. If you look at the BGA ladder for the various clubs mentioned, the recorded flights will give you an idea what might be possible at that club at the time of year you are looking at.
PS you might also want to look at the possibilities at Eden Soaring, near Penrith. I believe they have the longest ridge in the UK!