Hey, I happen to be in the UK right now with my Trophy, so maybe I can help you out.
First of all, I'm not sure about different version numbers for the radio, but I assume everything should work if you've done the updates for which you were notified. I did the same thing.
Call Verizon before you leave and ask to speak to the "global" department. Tell them that you're going to the UK for about a month or whatever and that you need your SIM unlocked. I had to call three times before I actually got someone who knew what I was talking about. The first two people told me that my phone would just work when I get there, which is completely wrong. When you find the right person, they will send you a text with the SIM unlock code. There is an individual one for every single phone, so if you have more than one Trophy you need more than one code. This code is very important because when you put in a British SIM it will ask you for the code. The good news is that once you enter the code once, your phone is now unlocked forever, allowing you to use it on any network you want that uses a SIM. (Verizon will not tell you this because they aren't really happy about it.)
I would keep HTC Connection Setup installed on your phone. When I put in a prepaid SIM that program automatically opened and configured everything properly. I don't know if its necessary (it may be), but it seemed to help.
I asked around and decided to go with "3" network. 3 sells a SIM that lasts for 30 days with 300 minutes, 3000 texts, and UNLIMITED data. It has been very reliable for me (in the Cambridge area). When you put the SIM in you get a local UK number, so you won't be able to call or text international numbers with that card. You can always use Skype though with the unlimited data.
Overall, I'm very happy with my Trophy and it made my trip so much easier not having to buy a new phone. Good luck!