You're need to supply the IPS tag, EPP tag doesn't exist on .co.uk or any other ccTLD uk domain. However the registrar may allow you to insert the IPS tag into the same field as the EPP field, but again this doesn't make them the same.
What is a IPS Tag ?
Internet Provider Security tags are used by Nominet to identify the hosting company linked to a specific .uk domain name.
What is EPP code?
Domain Authorization Code (also referred to as an Auth Code, Domain's
secret or an EPP Code) provides an extra level of security for the
domain name registration. The authorization code is unique for each
domain name and is assigned by the registrar at the time the domain
name is registered. It is needed when you would like to transfer
domain from one registrar to another.
EPP authentication code will be required for .com, .net, .org, .biz,
.info, .mobi, .asia, .ca, .cc, .co, .cn, .de, .in, .me, .tv, .us and
.ws domain transfers.
Also, looks like you missed the documentation on Dyna's website in regards of transfering co.uk's domains.
How do I transfer my UK domain to Dynadot?
Step 1 - Please ask your current registrar (tag holder) to change the
tag (also known as the IPS Tag) for your domain to "DYNADOT". You do
not need to place an order on the transfer page.
Step 2 - Send us an email telling us your username, and the domain you
Step 3 - We will add the domain to your account, once Nominet notifies
us the domain has been re-tagged.
Dynadot does not charge a fee for this service. Please be aware that
your domain will not be renewed for 1 year, unlike domain transfers
for COM/NET and most other domain types.
If your current registrar will not or can not change your tag for you,
then you can ask Nominet to do it for you for a fee.