When I enquired (and am now Brand Registered). This is my reply from the Brand team:

"Note: Design-only marks and logos aren’t accepted for initial enrollment in Brand Registry. However, once your brand is enrolled, you can request to add design-only marks and logos to your brand.

To find the trademark type on IPOUK, select Names and addresses and then View owner’s other trade marks. Then, review the information next to Mark type.

If you see ‘Word’, your trademark is a text-based mark (word mark). If you see ‘Figurative’, your trademark is an image-based mark (design mark).

Then, select the trademark type under the Select the trademark type section of the Brand Registry application.

Note: Only word mark and design mark trademarks are eligible for initial enrolment in Brand Registry. Other trademark types can be added to your account once you’ve enrolled in Brand Registry."

I have bolded the important bit, so it might be that yours isn't the type of trademark that's accepted?

When I enrolled, I went for a word trademark and they accepted my Brand whilst I was awaiting for the authorisation form IPO, so I wasn't delayed by the month's waiting for IPO, as long as I confirmed the application number to them.

Hopefully this helps.