This could be the longest post in internet history, but I shall attempt to be brief,
In short, it depends on your employer. Most government departments give additional leave for reserve commitments, some larger private sector employers match this, or give one weeks additional unpaid leave to give more flexibility to the reservist. Few small employers can afford this. Assume you will have to use two weeks of your holiday entitlement for reserve activity until told otherwise. It's worse for the self-employed.
The MoD run something called The Armed Forces Covenant,. This encourages employers to support servicemen/women, particularly reservists. The MoD issue awards for the employers commitment:
https://www.armedforcescovenant.gov.uk/support-and-advice/reservists/
As for mobilisation (only every five years unless all parties agree otherwise) the employer has to keep you job open for your return, The MoD help pay for the training of your relief and a heap of other things. I could explain but its after 2300 at night.
In essence a mobilised reservist has the same rights as a lady on maternity leave, give or take. However (and her is the rub) proving that you have been discriminated against, deliberately or otherwise, a by the civilian employer, or potential civilian employer at ,selection/interview, is very hard indeed.
Finally note that we are required to inform our civilian employers, via units, that we are reservists. So you can't hide it.
That's it in a nutshell, but it is a very long topic. Brexit is easier!