You do not need to take any COVID-19 travel tests or self-isolate on arrival in England if you qualify as fully vaccinated. The testing and quarantine rules for international travel to England will change 11 February 2022. You can now choose a lateral flow test or a PCR test as your post arrival test.
Which prescription medicines are outright prohibited? Opium, cannabis and stimulant drugs (Amphetamines, methamphetamines), including certain medicines for the treatment of ADD/ADHD (such as Adderall and Dexedrine) are strictly prohibited and illegal to bring into Japan.
This means that, at least in theory, an overseas national can leave the UK following a full 6-month visit, re-apply for a visa from overseas and subsequently return to the UK, even if this is just a few short weeks later.
Although the situation has improved dramatically in recent decades, toilet paper is not always provided in public washrooms, in which case it can be useful to carry a small package of tissues with you. Similarly, because paper towels or dryers are not always provided, it is recommended to carry a handkerchief.