Rugby, Darlington, Salisbury, Winchester, Durham, Lancaster for instance, or say Northampton where service extension would come at a huge cost.
Lancaster bus station is utterly pointless. It's in the wrong place, not only away from the station but also away from the main shopping area too, it harks back to National Bus Company days and an attitude of "we need a bus station, here's some land, put it here then". The best thing to do with that would be to move a few services where it makes sense to cross-city operation to reduce layovers and make the stops on Common Garden St the main "bus station", this being a suitably short walk from the railway station and the closest you could sensibly get a big enough facility to it. It's a small city and doesn't need a large bus station, and it isn't even that nice due to vandalism etc. Sell it off and have a student accommodation block built there instead - that way you get some money from it rather than it being a constant and pointless drain on the taxpayer to maintain it. If Common Garden St wasn't enough you could add a few more stops/layover bays on Spring Garden St, it's wide enough.
The rest of your list are interesting as they're ones where the railway station is notably out of town which is always going to be an issue. I think I'd suggest with these that getting as many services to cross-city as possible is the best you can do where the local railway station is particularly inconveniently located.
But there are plenty of places where the railway station
is central, and so not having a combined facility is tantamount to vandalism against integration. Preston is about the best example of such gross stupidity - not only are the railway and bus stations something like a mile apart, but the bus station is less central than the railway station. The redevelopment was a prime opportunity to make the bus station a public event space and create a new smaller one (for regional services only) by the railway station. Planning gain as part of the Fishergate Centre redevelopment would be another great chance to get it for nowt.
Milton Keynes is an example of somewhere that never really needed one - cross city running along the Midsummer Boulevard spine with two smaller facilities, as it is now, made much more sense than the central bus station which was inconvenient for the shopping centre. Interestingly regarding my Preston comments MK Central Bus Station (the building is still there) is now to be extended and turned into...an event space.
There are also cases where (and this was done years ago) you can make a slightly awkwardly located railway station and a more central bus station link up OK, e.g. by dedicated paths, e.g. at Ormskirk.
Chester is another one where the railway station is inconveniently located so wouldn't make a good bus station, but the actual bus station is also inconveniently located. I suspect there most people board/alight buses at the stops on Foregate St. It basically serves no real purpose and might as well be redeveloped; if you need layover space you could send them down to the railway station.