I've done the home test - really easy and the results came back quickly (less than a week IIRC). I figured that there is certainly no downside to doing it and it could provide information my consultants might find helpful. I've recommended it to others. This was the site I used:
https://groupbstreptest.co.uk
That said, the latest UK guidance seems to suggest that you'll be given antibiotics automatically if you deliver before 37w, so GBS might not matter either way for you from that perspective if you do deliver very soon:
https://gbss.org.uk/info-support/pregnancy-and-birth/uk-guidance-about-gbs/