Why are people who have been double jabbed still going to hospital?

Having to isolate has nothing to do with the effectiveness of the vaccine, and doesn't necessarily mean you're suffering any symptoms.

As a fellow member from the UK, I can tell you this is just the result of poorly planned policies. If you're double vaccinated, you still have to isolate if you get "pinged" by the NHS App, that is until mid-August where this will no longer be a requirement. The reason being that the government are trying to wait until more people are vaccinated so that the chances of another increase in cases is less likely, and less severe.

However, don't think this means the vaccines are ineffective. The fact there are much fewer hospital admissions and much fewer deaths tells us the vaccines are having the desired effect.