Re-sitting A-levels externally (bio, psych and rs)

Ok I'm sorry for making this when there are a few similar threads but I have some specific questions. I basically want to talk to people who have resat their exams externally as I can't really find out that much about it for some reason.

I basically failed my A2 year dues to personal reasons and taking a fair bit of time off and not being motivated blah blah blah

These are the grades I got:

Edexcel Biology: D (B at AS level)
OCR Religious Studies: D (High C at AS)
AQA Psychology: D (med-low C at AS, I find it quite a hard subject)

I'm pretty confident I can do a lot better, I'm aiming for AAB-ABB, but I don't want to go back to college as I don't see much point when I know about 70% of the syllabus for each subject so I don't want to just literally repeat the year again. I've gotten A's in mocks in bio and RS, and I plan on working a lot to get better at writing psych essays.

Has anyone resat edexcel bio? Did you do it at your old college/sixth form or a private exam centre? If so how did you get your core practicals marked? (I didn't even know they were marked until I looked on the website a couple of days ago). And did you have to pay for resits?

Also if you resat at a private exam centre I don't suppose anyone knows of one in Essex/East London?

And do you think it's worth it to take up another AS level or something? I'm thinking about taking evening classes for a language as well.

Do you think it's worth it to resit any of my AS exams? I got B's in both biology AS exams and a D in my coursework but I plan on redoing that (for the 3rd time -_-)
I got a C and a high B in RS so if I get B's then it will average out to a B anyway so I don't see much point resitting the C.
For psych I got a C and a D so I'm considering resitting the D.

Sorry if this isn't really clear I'm really tired atm. And please don't say that "if you got D's then can you really do much better blah blah" because I know I can do better if I actually revise this time.