According to the reasons I outline in this answer, the best options seem to be:
Most natural British English: email or post.
Most formal British English: postal mail or email.
Best for germanic language natives: email or post.
Best for British and trans-European English: email or postal mail.
Why not "mail or email"?
"Mail or email" is not a good solution in my opinion (for British and European English speakers).
In British English, which the OP is asking about, "mail" is already weird. "Post" is the word that we use in everyday speech to specify postal mail.
For Europeans who have learnt English, "mail" would be very easily confused for "email". Not just because they are so similar morphologically, which makes it hard to distinguish them in the first place, but also because of their native languages: romance languages seem to slowly be giving up on distinguishing between "mail" and "email" (even more so than English); and "post" is a germanic word, so German natives, Dutch natives, etc., will find anything containing "post" easier to notice and understand than when it doesn't appear.
"Mail or email" is also weird, and that's for English in general — not just British English! In these kinds of contrast-patterns, readers expect like-nouns (in this case "mail") to have contrasting qualifiers. Perhaps this idiomatic pattern has arisen from generations of writers intentionally disambiguating the two sides. "Would you prefer to be seated in a carriage or a club carriage?" is very weird. Even though "carriage" can normally be used alone for exactly the same meaning, in juxtaposing cases like these it's expected to add the qualifier explicitly: "would you prefer to be seated in a normal carriage or a club carriage?"
To betray this expectation is a poetic technique, so it's not necessarily incorrect to say "hey, would you rather sit in a chair... or a deluxe ACME-pro gaming chair?" But it is a stylistic decision that has to be intentionally made, and in "mail or email"... are you sure your readers will appreciate the poetical surprise? Or even be reading slowly enough to notice "postal mail" is an option in the first place?
Any solution which uses a "post" qualifier to contrast from the "e-" qualifier in "email" would suit far better than just "mail", so e.g. "postal mail and email" is a great solution.
"email or X" vs "X or email"
Because of the way that we are naturally inclined to organise stress in English speech, "email or post" and "email or postal mail" sound more fluent than "post or email" and "postal mail or email".
"Email or post" is pronounced with a dactylic pattern: email or post. This rolls off the tongue, sounding rhythmic, casual, and non-agressive. In everyday colloquial speech, we'd likely say this.
"Post or email" however is harder to pronounce. If pronounced trochaically, "post or email", it sounds curt. Any other pronunciation, for example "post or email" (the most likely one imo), reassigns syllabic stress, either sounding muddier (less well-enunciated) and/or violating the standard stress pattern of the words, which also deteriorates clarity. By the way, the trochaic pronunciation "post or email" may be desirable, especially in a formal setting: somewhat stilted, carefully-enunciated speech is a common sight in formal settings. This might be because it dispels laziness and casualness.
Similarly, "email or postal mail" is naturally a dactylic dimeter ("email or postal mail"), that has the same positive connotations as before. It also agrees with the isolated pronunciation of "postal mail", which is a dactyl ("postal mail"), because "postal mail" is one conceptual unit so outside of metrical context it would tend to be pronounced as a whole. In some contexts, "mail" can be a stressed syllable ("the postal mail was slow today" — very satisfying iambic tetrameter), but in your case it's at the end of a sentence. "Postal mail or email" has the same rhythmic consequences as "post or email".
Whether or not anyone will notice (let alone care) outside of poetry is another question; this metrical effect could reasonably be considered negligible, especially in writing, especially in written communications.
So, my favourite solution to your question would be the totally unambiguous at-a-glance "email or post" that also uses the idiomatic most common words and ordering (for British English). It's the natural British English phrase.
According to a French friend who has learnt English in France (and online; admittedly, the internet is quite Am.Eng.-saturated, but learning through online immersion is more than common enough to matter these days), "post" is weird, so depending on your audience you may prefer to use the totally-specified "postal mail". "Postal mail" also sounds even more formal than just "post".