I understand the discomfort you must be feeling right now - but I'm afraid these are really questions you will have to direct to your new employer. But that said there are a few things we can clear up:
Can my new employer move my start date back? (if so will I have no income for weeks/months?)
This is a grey area - if they've already put your start date in writing (be it in an email or hardcopy) in an offer or contract (in this case the job offer essentially forms the contract of employment until they have you sign the actual one) they can only really put it back by mutual consent. If it's vaguer then it's going to be trickier.
Can they revoke the job offer?
Technically no they can't - assuming the offer was either unconditional or that all conditions have been met by yourself then the offer is binding. But what they can do is essentially let you go immediately and only be on the hook for the minimum amount of notice that is required by the contract. Since you don't have a seperate contract yet this would fall back to the statutory minimums, which is a week.
Is it possible for me to work from home at a new job?
This is entirely between yourself and the new employer - the current exigent circumstances are making this very much a hot question amongst employees and employers alike. The official guideline to "work from home where possible" was only announced late yesterday so not everywhere has sorted things out fully yet.
how can I ask these in a way which won't jeopardise my new working relationship and/or see me without income for the next few weeks?
The best approach is to ask these with the presumption that the job will be going ahead as planned, you're expecting to start working for them on Monday and ask how they want to proceed. One thing that will be helpful to remember is that everyone is in uncharted territory right now - and if you can approach this with the attitude of working with them to resolve the challenges that's generally better than simply stating questions or demands so if you can bring suggestions all the better e.g:
Given the current situation and the government recommendation to work remotely what's the company currently doing? How do you want to approach starting on Monday? We could have a teleconference/skype/whatever
You get the idea.