As Schrodinger says, it's worthwhile looking at the sentencing guidelines for the offence.
Basically, any judge has to stick to the guidelines.
For this particular offence you can read the guidelines on page 45:-
www.sentencingcouncil.org.uk/wp-content/uploads/Feb-2016-Sexual-Offences-Definitive-Guideline-web.pdf
The starting point for this offence is 5 years in prison with a range of 4 to 10 years depending on mitigating or aggravating factors.
People have mentioned about other cases where the offender got a much lower sentence. Again, that comes down to the guidelines and where there is a lower level of culpability then the starting point is 1 year in prison with a range of a community order to 2 years in prison.