A few double-decker buses do not have the is_tall parameter set in their .dat files. Namely, AEC NS-Type, AEC K-Type, Leyland K2, MCW Q1, and Leyland Atlantean AN68.
I have fixed these and started a pull request which can be found here (https://github.com/jamespetts/simutrans-pak128.britain/pull/62). This should be an easily incorporated pull request.
Title is misleading. Double-decker busses are not meant to pass under low bridges so if "A few double-decker buses cannot pass under low bridges" the behaviour is working as intended. However by the sounds of your post you meant to say "A few double-decker buses can pass under low bridges" which is unintended behaviour.
Thanks for pointing it out. I've amended the title to something clearer.
Thank you for that: I incorporated that a little while ago but forgot to acknowledge that here. The fix is most helpful.