The problem is UK law (as usual)
The problem with giving away old computer equipment in the UK is basically the law. As many posters have pointed out, if you sell old equipment, you are liable to tax, thereby needing to pay and expensive accountant to sort it out.
Having Windows on it will also fall foul of licencing laws but that can be got round by using Linux but that will cost money as someone will have to be hired to reload all the PCs.
If a gifted PC then blows up, catches fire, electrocutes or injures some beneficiary, the donor could be sued. Therefore each PC will need to be checked and PAT tested, again requiring a hired hand.
Sending them to third-world countries is not the answer either as the cost of doing this is quite prohibitive, not to mention again having someone to check them over though if for other reasons rather than legal.
The answer? If the government is really serious about green issues and recycling, the law should be changed to accommodate the above. Sure, the treasury will lose a load of tax as I am sure smart entrepreneur will use the bound-to-exist loopholes and make themselves a fortune.
I'd better get my coat as I can hear the bean-counters and health-and-safety lobby bleating in the wings, drowning out the practical and logical creative-thinking individuals.