Trucknet UK comes up on Google listing several discussions on here answering your questions so use the Forum search engine. GOV UK site lists O licence criteria and exemptions for recovery operators. The DVSA Moving On blog as well has advice answering questions about what you have to be careful about legally : distance to job, tachograph, digicard, what constitutes haulage, what legally defines a recovery vehicle etc. If you need an O licence then you need an operating centre. What they don’t tell you is that you just might be able to carry a fully loaded 3.5 t van on a Japanese chassis slidebed legally, otherwise the answer will almost certainly be no on a 7.5 tonne vehicle.
I’ve got one, re registered it from regular Hgv classification, but not a recovery operator, do predominantly transport but it’s useful for some of the stuff I do, bearing in mind the restrictions of use if its not on o licence, which our one isn’t for various reasons