Sorry to jump in on this thread, but I'm going through a similar process now, albeit not quite the same. I have money in a GIA, it's been there since around 2000/01, the organisation it's with don't have much of a choice of funds, so I figured I'd withdraw some of it at the start of the next tax year and use it to fund my ISA with a different provider.
I did think that an easy way would be to sell out £12300, so that cannot possibly all be gain and I definitely won't have any CGT to pay, but isn't there a need to declare the capital gain on my self-assessment, though? I'd still have to work out what I've gained, and that's the bit I'm struggling with. All mine was bought at the same time (just before 9/11, which was great timing on my part) but several of the fund names have changed at least once over the years. I have paperwork, masses of it, and I guess it might be in there. I was hoping that when I sell, the organisation might put it on the annual statement, but I haven't asked them.