This is what I'm trying to do!! Like you I did grade 5 in November and wan to carry on with theory, but can't really find anyone/anywhere to teach grade 6!!
I've got the ABRSM books (Music Theory in Practise, Workbook and The AB Guide to Music Theory) and am slowly working my way though. So far I've kind of got the hang of figured bass and writing chords to harmonise a melody...the only problem that I've found is that no one can check my work to see if I'm doing it right!!
I think what I might do is work through it myself (which seems possible, although hard, especially as I've got a stupid amount of school things to do as my exams are coming up!) and then do a couple of practise papers and see if I can run it by my music teachers at school or something...
You didn't say how old you were, but if you're at a-level age, then you may be thinking of taking music at a-level, and would learn a lot of grade 6 theory there.
Good luck - I'm sure you'll be able to do it so long as you keep at it!!

Laura