It is worth getting a few certificates to give you firm evidence, and they are not all that expensive - the General Record Office are doing a pilot scheme supplying PDFs of birth and death certs for £6.
A birth certificate gives you details for both parents, including a maiden name for mother, and an address. From that you can move on to the nearest census for that adress and those names. A death certificate will have an informant and their relationship to the deceased, a marriage cerificate had fathers' names and occupations and full nales, ages and addresses for both spouses. Well worth £6 each.
Do your searching of the indexes first (FreeBMD has birth, death and marriage indexes for England and Wales, free to search) and note the reference numbers of what you find. If you can't quote these, the GRO will charge extra.
You do know that you can use library computers to consult Ancestry without paying for a subscription? The libraries have a bulk membership.