Without the requested information, I can only provide you with some general information on this topic. Hopefully, it will still be useful to you. First of all, I am sorry to hear about the issues brought up by this. It must be a frustrating situation to be going through.
As far as the law currently stands, there is no legal requirement to self-isolate if you have tested positive for Dovid. Still, the employer may have their own policy which requires employees to self-isolate, just so they can keep the workplace safe and potentially avoid others there getting ill.
If it is their own policy which states that you cannot go into work if you have tested positive, then they should still pay you for the time you are off as it is their choice, rather than a legal requirement, and they should still honour your contractual terms.
In terms of being threatened with the sack, you could be disciplined if you unreasonably refuse to follow their policy which states you must remain off work if you are positive, although if you have been there for less than 2 years you have no unfair dismissal protection so they could easily sack you if they decide to.