For me if I go and use the public computer in my local library I can access the Ancestry website for free. However I do not believe this to be nationwide across the UK(?) and is obviously going to be different from country to country.
When I researched a distant relative who died in WW1 but joined the T.A. well before WW1 the earliest paperwork in his WW1 service record dated to 1911, when he joined the T.A.. The sad and poignant piece of information on this 1911 dated paperwork for me was the question about the applicant's employment. My relative had written that he was still in education. He had joined the T.A., in 1911, as soon as he was legally allowed to and was still at school/college. He died aged 20.
The good thing about the Ancestry site is that it has access to multiple archives in more than one country.