From what we got told, what it boils down to is that you simply cannot get a letter of freedom to marry from the UK as they don't exist. ...the longer explanation we got from that was that there was a specific piece of legislation that both countries would've needed to sign up to around Easter 1916 - no surprise, neither country got round to it.
What you can get is a ''no trace lookup'' from GRO. You can ask for it from their website or over the 'phone M-Sat. Can't remember the hours on Sat, but Mon-Fri it's 08.00-20.00. It typically takes 3 weeks to get one, or you can pay extra to get it sent overnight. It comes out as a letter with a red official stamp on it, saying that they have looked between the date of x and y - basically any time you told them - that could be since your 18th birthday if you never married, or since your decree absolute if you were divorced. It goes on to confirm that there is no record of any/other marriage, which is as near as you can get to the letter of freedom in UK.
Try not to worry. If it helps, so far we've found that on the Irish side, with the exception of one lady who's been really good, a lot of the Irish side, particularly the helplines etc have been no help at all and just don't want to help with anything. In my case I'm marrying a French citizen and we're both resident here and with UK GRO and the French Embassy they've been great and when there has been things that we've worried on, they've gone 'don't panic, it's just the Irish system'. One of the ladies we've dealt with even told me she's seen brides nearly in tears from the bullying and threats of 'I don't care if they do it this way, you do it this way or else...' that they've gone through.
Hope it goes alright for you. - You're one step ahead of us. We only just squeaked in booking our appointment as we'd understood it was a case of get every single document in place first and then make the appointment - with an implication of we're around, it's simple, pop in, ...only to get told this week ''you're cutting it very fine'' and that appointments are usually gone 3mts ahead of the 3mts notice.