Ensuring that everyone is able to participate in worship and other church-based activities requires careful thought. There are several groups of people who may be excluded, not only wheelchair users but also people with limited vision or hearing, and those who have difficulty walking.
These are important considerations even if you are not aware of anyone with a disability wanting to use the building. Access limitations may deter people without you being aware of it, and the Disability Discrimination Act now places legal obligations on building managers to improve access to their premises.
Churchcare provides guidance for churches, together with links to other resources including a detailed self-assessment form from the Diocese of London. A more basic form from Truro Diocese can be found here.
Through the Roof is a Christian Charity whose vision is "for all disabled people to be free to fulfil their God given potential". Through the Roof offer advice and support to churches, in particular through their Churches Inc initiative.
Adapting historic buildings for disabled access can be challenging, but can be done very successfully. English Heritage's Easy Access to Historic Buildings is a useful document with examples of altering buildings to provide access. As with any alterations to church buildings, you should consult an architect and the Diocesan Advisory Committee or equivalent at an early stage.