NHS wheelchairs should be returned to hospital/clinic.
Wheels for the World, part of Christian charity Through the Roof, accepts used wheelchairs, for refurbishment and distribution to people in need in Africa. They can accept folding, manual wheelchairs and folding walkers; also crutches and walking sticks. Contact 01372 749955; email info at throughtheroof.org (Currently based near Salisbury, but moving to Aldershot)
Disabled gear.com has details about buying and selling manual and power wheelchairs and has links to many charities that accept used wheelchairs.
If in poor condition, take to Marlborough Household Recycling Centre and ask where to put it (probably in metal recycling).