You can move a domain out at any time — there's no fee and no notice period. What you need depends on the ending.
.uk domains (.co.uk, .uk, .org.uk): tell us on a ticket which domain to release and the IPS tag of your new provider (they'll give you a short code like ABC-XYZ). We change the tag, usually the same working day. One catch worth knowing: some receiving providers don't accept incoming .uk transfers automatically, so also start the transfer-in process on their side — otherwise the domain sits at "pending handshake" until they pick it up.
.com, .net and most others: in your client area go to Domains → My Domains, select the domain and choose Get EPP Code (you may need to disable the registrar lock and ID protection first). Give that code to your new provider, then approve the email the registry sends. Transfers usually complete within five days.
Before you go: if your website or email is still hosted with us, the domain's DNS will need recreating at the new provider or pointed back at our nameservers, or things stop working the moment the transfer completes. Ask us for the records if you're unsure — we'd rather you left tidily than went offline.