The best solution for most people would be to book the following:
- Carlisle to London as a single journey.
Break of journey; have some time in London
- London to Edinburgh as a single journey
Break of journey; have some time in Edinburgh
- Edinburgh to Carlisle as a single journey
If you want to compress it further, then book Carlisle to London as one journey, and London to Carlisle as another journey (specifying via Edinburgh & Glasgow; a good retailer will ticket this, such as the forum's site)
However I think that given current reliability issues and daily chaos, the open return option with an almost guaranteed delay repay refund will suffice.
Delay Repay isn't technically a refund; it is also not guaranteed by any means.
Also you need to decide what your journey is as this is important for calculating Delay Repay; you would also be on shakey ground if you became delayed and then acted in a way that did not minimise delay and claimed for a greater delay than was necessary.
Plus I'm still not confident that if one advanced fare (say Avanti West Coast CAR-EUS) made me missed the next (say LNER KGX-EDB) I will have a leg to stand on in terms of getting the next KGX-EDB train.
You need to define your journey; providing you have a valid connection, adhering to all minimum connection times, you are entitled to take the next available services (but what I don't think you can do is act in a manner that doesn't minimise the delay
and go on to claim Delay Repay compensation!)
It may not be wise to state your journey is Carlisle to Carlisle, so think carefully about having a break somewhere.