The Stay at Home document says, inter alia:
Medical reasons
You can leave home for a medical reason, including to get a COVID-19 test, for medical appointments and for emergencies.
- so that's the plasma donation covered.
(Further reasonable excuses include - seriously - leaving home for the purpose of picketing. I must admit I hadn't noticed that one before. So walking with a drink in your hand 2 metres away from a friend in a quiet area closer to your doorstep than Stratford is from Downing St, = illegal, standing on a picket line, no maximum distance specified, on you go, then, lads. So perhaps the SU could plan a protest against something going on on campus? Everyone could go back then, with government blessing. But I digress).
The same document refers to the previous guidance to HE re students returning, which indeed says:
However, HE providers should consider supporting the return of students in exceptional circumstances, for example, where students do not have access to appropriate alternative accommodation, facilities or study space, or who need to return for health or safety reasons.