What was the travel market was like before the UK joined the EU?

Simple: There were no cheap flights. Mass tourism was only starting to get going in the late 1960s/early 70s, and charter flights tied to your package holiday were the only way to avoid paying phenomenally expensive air fares. Most people holidayed in the UK or maybe Ireland, this was the heyday of the seaside resorts and holiday camps like Butlins or Pontins, and you weren't allowed to take more than £50 per person spending money out of the country.