I appreciate that you must feel frustrated at the prospect of having to do a Degree, but all of us qualified Teachers have had to do the same.
I wanted to obtain a Degree, yet used to often feel quite annoyed and frustrated whilst studying for it, as I was so desperate to become a Teacher and wanted to fast forward time! If you could study part time, yes it would of course be very demanding, but it would be great that you could remain working in your current school. You would also be noted for your dedication and commitment.
I taught in an incredibly challenging Academy at one point, and two of my colleagues in my department were unqualified Teachers, as was at least one other person in a Teaching role in another subject area. As well as not holding a PGCE or equivalent, neither of the 3 held a Degree, but had experience of working as TA's in the past. They worked incredibly hard, and were very dedicated, but after Ofsted had graced us with their presence, the school went straight into 'special measures.' A new Principal swiftly took over and made plenty of changes - one of these was to advertise the positions of the 3 colleagues. 3 Qualified Teachers were employed, and the 3 unqualified Teachers were re-employed there as TA's. They had been paid on the unqualified Teacher salary, and so their salaries suddenly dropped, and their pride and esteem felt dented. They were trapped however, as all 3 applied for several Teaching positions elsewhere and failed to be interviewed, despite several years of experience teaching, but as they were not qualified, nor held a Degree, other schools were not interested.
The 3 non-qualified Teachers were not very popular amongst many of the qualified staff and received quite a backlash. I had a lot of respect for them, but ultimately they were used by the system (paid more than a TA and allowed to Teach, yet paid a lot less than qualified Teachers, not able to gain a promotion, and then 'dropped' by another Head). It is a completely unsafe position in terms of security. I would always advise anyone against employment as an unqualified Teacher, as I do think it places people in awkward situations.
If you obtain a Degree, you will then have a greater flexibility for the future, should you wish to change careers, and with QTS, you will have security and be able to seek promotion should you wish to.
Do you live in the London area? If so, Birkbeck University are fantastic. All of their courses are delivered at nighttime, with the earliest classes being taught at 6pm I believe. You can study part, or full time. They are a very flexible and understanding institution. If you can't access Birkbeck, I agree with other posters that the OU would be a great idea.
I wish you all the best in pursuit of your career. You are very lucky to have such a supportive Headteacher who values and appreciates you - this is becoming more and more rare, as is his offer of assistance. He must be very impressed with your work. Be proud of yourself and take his offer and support as a huge incentive.