They do it in Swindon with no apparent problems, but there are relatively few motorbikes or cyclists here; it's also not necessary to use bus lanes to get anywhere if you're cycling, because there are alternative psycle routes, and you see few cyclists on the bus-laned roads.
There are cycle route alternatives to Finchley Road, The signage is intermittent, the route full of turns and so hilly that this is a snip from my David's discussion of it Elsewhere.
<< Same as above. You can turn off at any of these places, or at Platts
Lane, but only if you are seriously in training for something
involving mountains would you use this route to access West Hampstead
or Fortune Green from the south>>
Hampstead is not flat but Finchley Road's gradients are tolerable and the bus lanes make cycling OK. My experience of PTW was that I found some of them did intimidate. It might only be a small minority, but those are the memorable ones.
Unfortunately their over-representation in the KSI stats deters me from wanting to share.