There are plenty of UK services providing numbers for an incoming UK number. but with some of the main problems being the costs associated with the services.

Downsides to a UK Number
The user calling you has to pay a fee of up to 20 cents on their phone bill to call your number. One reason your friends might not be so keen on calling you at the number.

Getting a Free UK Voip Number
Two new services are offering UK voip numbers.
1 UK Number
2 UKnumber4U

These two options both work but keep in mind the costs of the user calling.