Domains registered under .co.uk do not use auth code/EPP code for transfers. Each domain registrar instead has a unquie IPS-TAG for identity. To change registrar, the current registrar must update the tag to the new registrars IPS tag.
To transfer a co.uk to AVATAVA/Tucows the IPS-TAG to use is TUCOWS-CA. To transfer a co.uk domain away from AVATAVA/Tucows, you need to contact hostmaster@avatava.net with the domain name and the IPS-TAG of your new registrar.