Firstly if you look at the original posters other threads it would appear he has experience back home in the States, so may not be a 'new' PPL.
However, I have to correct his statement that SkyDemon does show unflyable conditions. This is from the manual:
Flyable Conditions is a personalised forecast based on your own ceiling and visibility preferences and is designed to help you visualise the weather conditions to give you a quick idea of where the weather is not good enough to go flying. It's a short-term forecast and you can look up to two hours ahead, as well as a few hours into the past. This weather overlay looks at the forecast cloudbase and visibility, and combines it with your preferences to create a useful coloured representation of where the weather is predicted to be good, marginal and most importantly, bad.
This subject came up on the main SD forum recently when someone asked if the feature could report actual cloud bases AGL in dynamic time. Obviously not, as it's only a forecast and entirely dependent on what criteria you pre-load.
As far as what cloud base (AGL) to use in low lying areas of the UK I would suggest a minimum of 1500' unless operating in the higher ground areas of Wales/Lakes/Pennines/Scotland where 'mountain flying rules' are more appropriate. A simple Google search on the subject produces a lot of useful free information.