Hi EllieM, That is a difficult decision. I presume that you would normally get a flu jab and that because of your diabetes you are more at risk than the average person should you catch the flu.
In the UK currently GP surgeries are relatively empty as Doctors are making use of telephone consultations. So the risk from sitting in a GP surgery is lower than it was a couple of weeks ago.
Also in the UK, pharmacies also did the flu jab as well as GPs. I got mine at my local pharmacy in October, we had to book it and as it turned out we only saw 4 other customers in the pharmacy during that time.
You say that you are self isolating. This can be a little vague are you still doing food shopping? -Or do you have a delivery? If you have a delivery, is it one where you and the person delivering remain at least 2 meters apart?
If you are sticking to the 2 meter rule with everybody, and are wiping down all items delivered, washing hands after touching things etc. then you are very unlikely indeed to come into contact with either flu or Covid-19.