The proper way to do it, if you are eligible, is to claim transfer of residence relief, where you should obtain prior approval from UK customs and obtain a reference code, which you should provide to the clearance agent (UPS for example).
If you are approved for the relief, most personal items may be declared under "Personal property belonging to natural persons transferring their normal place of residence" with commodity code 9905000000. Writing "personal goods - transfer of residence" is probably fine if you refer to the commodity code.
If you really just have a couple kilograms of clothes (that are not expensive or special, e.g. special fur or animals products) and non-electronic office supplies, and you do not want to bother with the relief, usually the requirement for declaration is not so strict. For clothing, a general declaration of clothing sometimes can be enough for personal shipments, but sometimes you may be asked to be a little more specific ("T-shirts - used", "Men's underwear - used" etc.).
For the estimate, keep it reasonable, you do not need to declare the purchase value; most of the time the customs would not spend time to evaluate your clothes just to get 30 pounds instead of 10 pounds in taxes and duties.
What happens if I mess something up on the form?
Unless it contains something illegal or subject to special regimes (e.g. alcohol, tobacco, special animal products), the worst thing that can happen usually is that it would be returned to sender or you may need to pay some duties and taxes as estimated by the customs.
