There are, as many people have observed a whole bunch of possible causes for this, but from experience I would say check out the simplest options first - and the simplest option is that the battery, whilst holding enough charge to run car electrics simply cannot deliver the 400 Amps plus that is demanded by the starter. So options are 1. Try Jump starting it from another vehicle or 2. if it is a stick shift (as most UK cars are) you can try bumping it! On my Toyota the battery frequently dies because I have a nasty habit of leaving the interior lamp on.
Other possibilities include faulty earthing as suggested, or sometimes if your starter is of the non pre-engaged type a sticky bendix gear - in which case again the solution is either for an auto to give the starter a sharp rap - or if it is a stick shift place the car in reverse and rock it FORWARD which will dislodge the gear.
Finally and most expensively the motor or solenoid or indeed the security or engine management may be out - but in my experience this is usually not the case. 90% of the time the solution will be found in one of the simpler solutions.
As you can gather the engineer in me is still very much alive and well even after 30 years on estrogen. I am happy to report that in 90% of cases I can usually fix my own and/or my neighbours cars. The computerised systems do make things different, but it is possible even as a hobbyist to get the diagnostic rigs and harnesses at affordable prices - then having done so a lot of problems can be fixed using a laptop and the appropriate downloads. I remapped (chipped) my diesel using this technique and the resultant performance increase has saved me a fortune in fuel.
In terms of cycling and being a non car household I agree - we are now, for reasons of cost, trying to cut down our usage too, and we have not one but two cars rusting in the garage.
Good luck anyway. I'm pretty sure you will find that the fault is actually quite simple when you get a chance to really look into it.
