To transfer a .uk domain to another registration company please see this article
If you wish to transfer a non .uk domain to another registration company you need to follow these steps:
1. Log into the Host100 Client Area at https://billing.host100.co.uk/billing/clientarea.php
2. Click on the 'My Domains' link
3. Click on the green arrow of the domain you wish to transfer
4. Click on the 'Contact Information' button at the bottom of the screen
5. *** This is very important ***
Make sure that the 'Registrant' and 'Admin' Email addresses are set to your email address. You may find that these are set to Host100 details as these are sometimes used as the default. Once you have updated the details then click on 'Save Changes' and then click on the 'Back' button.
6. If the 'Registrar Lock' box is ticked then click on it to remove the tick and click on the 'Save Changes' button immediately below it.
7. Click on the 'Get EPP Code' button at the bottom of the screen.
8. You will then be sent at automated email with a code in it. You need to give this code to the new registration company who you want to transfer the domain to.
Once your new registration company has begun the transfer procedure you should receive another automated email asking you to confirm that you want to transfer the domain, with a link to a web page where you can confirm this action.
After this the domain will usually be transferred to your new company within around 5 days.