Hi,
I've just opened one of the Halifax International Regular Saver accounts - at least I've sent the documents to the Isle of Man.
As yourself, I was a bit worried about the safety of my savings, but rest assured, all amounts up to 50k£ per individual are guaranteed in the Isle of Man, which (I think) is more than the current 35k£ in the UK - at least it's enough for my little wealth, so from a safety point of view there's nothing against them.
However, I found the need to provide them with certified copies of passport and proof of address was a bit odd, given that I am with Halifax already. Nevertheless, I just needed to pop into a Halifax branch and get a rubber stamp on the copies from one of the staff, so apart from that it was quite easy. Let's see if everything works well...
My main advantage of Halifax International is the higher maximum monthly deposit - I don't know why you're capped at 250£ per month for UK banks, but with them I can put a lot more in each month - up to 2k£ I think. Otherwise with the 250£ limit you might have to idle your money to pay it into the account month after month, and so you will lose out on interest - probably thats the reason why the UK banks do it though ;)
Anyways, I will report once everything is through, but don't worry too much, it was a piece of cake ;)
Regards,
Mat