Not an app, but since you're in the UK, you may find it useful to get a subscription to the
Met office. Their free subscriptions give you access to METARs, TAFs and the 215 and 214 Forms.
Practice reading and understanding them - you will be using them soon and will continue to use them once you have your license.
If you start to read/understand them as soon as possible, it will also help you work your way through the Meteorology subject.
You can try to use SkyDemon as a practice tool to check whether it gives the same answers (in terms of GS and headings) as you get after working out a route by hand, but, as mentioned, don't get tied to it.
The best 'app' you can get for navigation is a
manual chart. Read it and understand everything on it and you will always be comfortable using it - whether it is paper or digital.
Most importantly - enjoy the flying
B.