A GBH charge, brought under Section 20 of the Offences Against the Person Act 1861, carries a maximum sentence of 6 months imprisonment (plus an unlimited fine) if dealt with by a magistrates' court or 5 years imprisonment (plus an unlimited fine) if the case goes to Crown Court.
A GBH charge, brought under Section 18 of the Act ('with intent') can only be heard before the Crown Court, and carries a maximum penalty of life imprisonment.
Affray can be heard either a magistrates' court or the Crown Court. The maximum sentence is 6 months or 3 years imprisonment, respectively.
There's insufficient information in your post for an accurate assessment of the likely sentence but many cases of GBH+affray, reported in the press, seem to result in sentences around 18 months to 3 years imprisonment. (if the offender has no previous convictions then he/she might</> avoid prison, and receive a community punishment, but it's probably unlikely).
Chris