I can't answer all the questions because .com uses a diferent host. I can give some general answers only and perhaps Matt can give you more specifics about Bluehost.
4 of my websites are large businesses here in the UK. What would be the downtime in tranferring?
You do not necessarily have to have a lot of downtime. You can copy all the files over using FTP, then do installs, configs, and other setups while you continue to run from the old host. You will need to have a little downtime to get the DB in synch, which should not take very long it you plan well. Then just before you come back up you change the DNS to point at the new site. The DNS will take some time to propogate so you can use a re-direct to handle the transition.
How does this work - running 6 hosts on 1 account. Do I have 1 IP address?
Shared hosting means shared ip, unless you pay for dedicated ips and then you can uses the DNS to assign which sits go on which ips.
Is the bandwidth split up between the 6 domains?
Bandwidth quota is at the account level.
Do each domain have its own control panel? etc etc
Probably only one CP, and a single file system.
The other thing you need to look at is how much CPU you will use. The cpu quota will be at the account level, so if the sites are busy total cpu might be more of a limiting factor then bandwidth. If you use too much CPU, a host will gernerally require you to upgrade or they will suspend the account. So make sure that your total requirements for all the sites is not going to be excessive.
Low cost hosting companies generally oversell capacity, and that can result in response time and reliability issues; and because you are sharing resources with other accounts, you can be impacted by abusive use on your server by another user until the host takes action against the offender.
