Why? We never had to get them before the EU.
They will still want Sterling every bit as much as they want it now.
If it does introduce a visa presumably it will be like the E-visa that you get for Turkey and takes a nano second to apply for. I can't see them wanting to stuff their tourist trade as it's one of the few things they have going for them.
They are unlikely to put themselves on the back foot compared to Turkey and Egypt who must be main competitors to their tourist trade offering cheap package holidays, and have risen to tourism success despite not being in the EU and the need for visas.
Sounds like you're putting out some propaganda here, and trying to scare people.
Really there are far more important things at stake than the British's ability to have an annual cheap package holiday or two which is after all not one of life's essentials but a luxury. Governments are more interested in controlling our overflowing borders, the inability of the NHS, Education system, infrastructure to cope and the ability to provide jobs for an ever growing populace; not you getting two cheap weeks on a beach (as indeed they should be).
We have had it pretty good for the last 40 years in that respect compared to most places - perhaps we should just be grateful for what we had, not mourn what we may not even be losing ! !
PS if they want to bring in visas for France and Benidorm I feel they should do LOL.