Assalamualikum, sister Hannah
I guess, as a non-Muslim country, UK doesn't have special rule for hajj travel agent.
But arrange a hajj trip is different than arrange a leisure trip, because there is quota for hajj who can come to Saudi Arabia during hajj season. If a hajj travel agent doesn't hold this quota, their customers will not get hajj visa. But there is no quota for umrah
Your dad can contact Saudy Embassy in London (30 Charles Street, W1J 5DZ - Tel: +44 (0)20 7917 3000) to get information about how to become an authorized Hajj/Umrah travel agent in the UK, and get hajj quota. They have consular service for hajj and umrah.
This is the web page of about hajj and umrah from Saudy Embassy website.