“We didn't use to go through an EU only section of passport control; one set up to make movement easier.”
No we didn’t. In practice, for arrivals from outside Schengen there is little or no difference in most EU countries between the procedure for EU citizens and that for non-EU citizens. The big difference came about for UK citizens returning home. They used to have a “UK Passport holders” section. Now they only have an EU channel. So instead of a queue comprised solely of their compatriots they now have to line up behind visitors from 27 other countries.
“Also at the moment. if you fly say from Spain to Italy with a British passport you don't even go through passport control when you land in Italy. Presumably in the future you would have to fill out a declaration form on the plane and go through passport control if you are British.”
No you won’t. Ignoring the “temporary” border controls invoked occasionally to deal with migration crises, there are no border controls for intra-Schengen flights (or any other cross-border traffic) within that area. There is absolute freedom of movement for all people once they have entered the area. These are the rules:
“For flights within the Schengen Area law enforcement agencies, airport authorities and air carriers are only permitted to carry out security checks on passengers and may not carry out border checks. Such a practice must only be used to verify the passenger's identity (for commercial or transport security reasons) and not his or her immigration status.”
There is thus no difference between the way EU citizens and non-EU citizens are treated and nothing like a declaration form would be required.”
So I will say again that in practice there will be no discernible difference in the arrangements needed for the vast majority of UK citizens travelling to Europe post-Brexit. Of course the EU could always place a visa requirement on UK visitors (leaving the UK out of the visa exemption list which includes countries such as Argentina, Bolivia, Ecuador, Panama, Parguay and Uruguay). That wouldn’t surprise me but I doubt it will happen. Currently visa-free travel is available from those countries only for trips of three months or less for those not intending to work. If that applies to UK citizens then I agree those (relatively very few) travellers who want to stay longer or work will see their arrangements changed. Other than that I will repeat that if you consider the current arrangements and those that will prevail post Brexit there will be no significant difference for the vast majority of travellers. Anything that suggests that travelling for your annual two weeks in Benidorm will be any more difficult is simply scaremongering.