From what I recall, you use the 'transfer away' option, select a reason why you are moving, then scroll down and pick the IPS TAG change option and submit. I think LCN take 48 hours to manually process it, at which point, as with any domain transferred in, you should get an email from Nominet confirming its arrival. Just make sure that if you have some kind of obscure privacy setting from LCN, that you untick it first, as your (assumed) self-managed tag from Nominet only allows 5 connected names and it might not match what you already have. By the same token, make sure your name is included in that list of connected names.