Time of sick

It's harsh but I think OP needs to seriously re consider his stance here re time off and GP certification.

Having time off for ' a cold' is rarely seen in a sympathetic light nowadays and with so little employment time and an earlier period of sickness I would proceed with caution.

To me that makes it all the more important to get to see a GP, anyone will do, do you have a walk in centre for instance? A GP certificate stating something other than ' a cold' will look better when any consideration is made about future employment. I have HR experience (in the past) and a self certification for anything longer than a day, two at most for ' a cold' would trigger an automatic employment review.

I am also very concerned by the statement in the OP 'I know I won’t be sacked or anything' which either means the OP knows something else that we don't, or else OP believes (as has been mentioned) that being sick protects you from being let go.

The reality also might be that if the OP has worked alone at home for a while they may catch any little bug or cold going once they work in a modern office with others. To many of us having colds and feeling awful at work for a period each winter becomes fairly normal . I was at work on Friday with several people who had very heavy colds and who were working , topped up with lemsip and day nurse, as normal. No doubt at some point this winter I too will get a cold (although the older I get the less I seem to get) and will sit on the tube feeling very sorry for myself.

I know we are not allowed to offer medical advise but OP, look at ways to increase your resistance to common colds, or at least reduce the symptoms, different things work for different people, some swear by Vitamin C pills or other vitamins, or just make sure you eat healthily and remain fit.