£275 to visit what is an annex to the Science Museum, which has free admission is outrageous! I don't know how long they have been doing this, but as far as I knew you could visit there at any time when staff were on site without charge. Suggest you wait until they have an event on there, like a vintage car or bus rally, which charges a nominal £5 entrance fee and then all the museum hangars are open for free.
This guide was only updated in July and admission was free then:
Science Museum - Wroughton