They have pretty awful swrvice but no other reason not to deal with them. They are registered with the UK compensation scheme so you are fully protected.
Now your ISA is up and running with account number it is now registered so no you cannot go and open another one elsewhere. You can however transfer your account to another provider as the registration for tax free status will go with it, I do not inow what problems will be created if it does not actually have any money in it.