I looked at this a few years ago, but quickly changed my mind. It’s interesting that no posters said that a small boat skipper could earn £10k; 20k or whatever. I think that unless you are very lucky that you will barely scrape a living. Ok for those of us with good investments or pensions, but...
It is tiring, especially in a small boat. Apart from all the sailing stuff, your body is constantly bracing against all the movements. I find a 20footer more tiring than a 40 footer for anything over a night in a decent wind. Keep at it!
No mystery, just look at any almanac or chart. But,no offence intended... If you have to ask this basic question, I would suggest that you are not ready for this passage.
If you plan Devon to Scotland return in 2 weeks with a 2 day stop every 2 days of sailing, you will be sorely disappointed. Start by checking the relevant tidal gates...
For 25k you could do a lot more. A depth sounder is essential, a compass, and a plotter/tablet. Sprurge the rest on restaurants and hookers... much more fun. Your 25k will depreciate like a stone anyway and you will not really sail any better.
I’ve always thought of competent crew to be suitable for previously non-sailing spouses or someone just dipping their toes into sailing. Someone who wanted to do CS would presumably be able to bypass this easily and save some cash.