[G-CPTN]

Most of the buses/coaches in preservation are kept on the understanding that they are 'not for hire or reward' and, thus, are not commercially available. This is a licensing/insurance issue (though a vehicle used commercially needs certification as such - which costs money as well as requiring conditions to be met).

Having said that, there are organisations that do hire out 'old' buses (and are certified to provide these services. You have to decide exactly what you (or your partner) understand and expect by a vintage vehicle - there is at least one 'charabanc' (an open 'lorry'-type bodied vehicle) and numerous immediate post-war era buses and coaches.
Maybe a London Routemaster would be what you seek and there are several of those about.

As this group don't encourage advertising (in fact it is prohibited) it isn't really possible to provide you with a list of possibilities, much less to find you a provider - however, choose your search phrase and use Google.
Terms such as:-
vintage coach hire
charabanc hire
old bus hire

or, if you have a specific make or model in mind (Bedford OB, Leyland Tiger, AEC or whatever) then you can add this to your search and you can also add your region and/or town.

Try several searches, including either bus or coach or double decker (open top if you wish).

If you want help in identifying the specific type of vehicle that you have in mind then there are plenty of members here that can help to refine your search terms.

You might contact your destination (Tenterden) and ask whether they know of anyone operating vehicles of the type your require.
Your local modern coach hire operator might also be able to put you in touch with suitable operators offering appropriate vehicles.