Many colleges of Further & Higher Education offer GCSEs, particularly in Maths and English, with many different time slots, including evenings and weekends. Now is a good time to get in touch with your local college and see what's available. You might be encouraged to take a less academic adult learners' course, but if your heart is set on GCSEs (and beyond?!), you can just try and push for those. There will also be lots of other adults taking part time GCSEs, and I highly doubt you'll be the oldest in the class.
Good luck!