Generally speaking, you can always transfer a domain you own to another registrar. Sometimes the losing registrar will charge you to do this, or hold you to a contract you may have signed, but there is often no technical reason preventing you from transferring a domain. A UK domain can almost always be transferred however there are a few reasons that a non-UK domain transfer is not possible, or fails.
The domain contact information is wrong.
If the domain contact information in WHOIS is incorrect, in particular the administrative contact, then the authorisation emails to transfer the domain will not be received and the transfer will fail.
Your domain is less than 60 days old.
If your domain has only recently been registered and is less than 60 days old then you won't be able to transfer the domain to another registrar. This is an ICANN policy designed to prevent some types of domain registration abuse.
Your domain was transferred recently.
A domain that has been transferred in the last 60 days cannot be transferred again to another registrar, i.e. from Registrar A to Registrar B, to Registrar C. 60 days must pass between transfers, this is an ICANN policy to prevent domain hijacking. The exception to this is transferring a domain back to an original registrar should a mistake have occurred and it was transferred in error or fraudulently.
The domain is locked.
A non-UK domain such as .com or .org can be locked in a number of ways, the most common being a domain transfer lock. If a domain transfer is attempted with the lock still enabled it will fail, disable the lock before attempting to transfer a domain.
The domain is in a non-transferable state.
If your domain is currently in the redemption period because it expired, is pending delete, or is on hold because of an abuse complaint, then it will not be possible to transfer it. In the case of the first two, the redemption period and pending delete, you would need to renew and reactivate the domain before you can transfer it. In the case of an abuse hold, you will need to contact the existing registrar to resolve this before the domain can be transferred.
UK domain is locked.
If a lock such as a phishing lock or abuse lock exists on a UK domain then it will not be possible to transfer the domain.