Medical
If you do not have an EU passport, it is advisable to obtain a medical certificate confirming that you have recently had a chest X-ray and are free from tuberculosis. If you do not have a medical certificate, you may well have to have an X-ray at the airport before being allowed entry into the UK.
Cigarettes
These are expensive in the UK so, if you smoke, you may wish to take some with you.
Packing
The weather in UK is notoriously variable, but take the absolute minimum of clothing and other items to reduce the weight of your luggage. Even if the airline allows you 30kg (66lbs) of checked baggage, consider the strain on your back as you drag your heavy case/cases on and off trains and buses, and up and down stairs when you travel between assignments. Make sure your case has strong wheels. Lay out everything you intend to take on a bed, then put half of it back in the cupboard!
Check the baggage allowances and security arrangements with your travel agent, airport or airline before you start packing. Both the number of items of ‘checked-in’ luggage and the permissable contents of your hand luggage change from time to time.
When travelling, it is often convenient to pack items to which you will need access (e.g. passport, tickets, money, reading matter, “freshen up” items, food) in a small, secure rucksack, as this leaves both hands free.
Take clothes that wash easily and are “practical”. When on assignment, you may prefer to wear slacks, both for comfort and modesty! Tee shirts make a good alternative to blouses. You may need only a few of the basic items such as Tee shirts, underwear, pairs of socks etc. Have lightweight shoes, ideally washable, for indoors and waterproof footwear for outdoors (the washing line may hang over wet grass).
In most towns, there are shops run by various charities (Oxfam, Salvation Army, Age Concern, Relate and so on) which sell donated goods at a fraction of the normal retail price. If you find that you have forgotten something, you may well be able to buy it in the UK.
If you are taking any electrical appliances, hairdryer, curling tongs etc., remember that the plug will not fit the UK, square pin socket. You will have to purchase an adapter or have the plug changed.
Finally, don’t forget to pack your sense of humour, you will definitely need it!