Interesting idea for a thread
There's generally a bit of conspiracy theorist stuff when it comes to these things (were services deliberately timetabled to be useless, or were passenger numbers so poor that there was no financial imperative to run as many services as possible with the stock? some lines were pretty untouched from the nineteenth century, so required a lot of infrastructure work to keep them going which may have tipped the balance in some cases), but some interesting examples nonetheless
Bangor-Caernarfon is certainly a good shout, there was a pretty good Llandudno - Bangor-Caernarfon bus service last time I went that way, which generally suggests a market where heavy rail could/should be viable
(Hull) Beverley - York is one that gets brought up quite often on "Beeching" threads - whilst the existing Hull - York service (via Selby) has never been particularly busy/frequent (and therefore, spreading the Hull - York passengers between two routes may just have resulted in two very lightly used services rathe than one slightly quiet service), I can see why it's a grey area though
Some closures seem "poor" by today's standards but I don't know what demand was like fifty plus years ago - e.g. the frequent bus service means that Tavistock to Plymouth seems worth investigating but commuting has changed over the past couple of generations and I don't know that there was the same market to sustain a train service back then, so I can't criticise the closure
I do wonder sometimes whether Beeching was right about the three lines from Manchester through the Peak District - he suggested closing the Hope Valley and keeping both the Woodhead and Bakewell routes open - BR took the opposite decision - there were merits both ways (but, now that the Woodhead and Bakewell routes are closed, I can't see them coming back) - tricky one