If the court believes that there was 'a significant degree of premeditation' (which might well be the case), that pushes the 'culpability' factor into the 'high' category. However a fractured kneecap would only make the 'harm' factor to be 'low'. That makes it a 'Category 2' offence, with a sentencing range (for a first-time offender convicted after a trial) of 5 to 9 years imprisonment. An early guilty plea can see the sentence cut by one third, but previous convictions (especially for violence) can see sentencing pushed higher.
So, at very the lowest end of that sentencing range, you're looking at 5 years less one third = 3 years 4 months. Of that, only 1 year 8 months would actually be spent 'inside'.
However if the court can be convinced that 'culpability' was 'low', it would only be a 'Category 3' offence, with a sentencing range of 3 to 5 years imprisonment. Taking a third off the lowest end, for an early guilty plea, would result in a sentence of 2 years imprisonment (of which 1 year would be spent 'inside').
Other than in VERY exceptional circumstances (which the judge must explain to the public in open court) a judge is not permitted to pass a non-custodial sentence.
See pages 3 to 6 here to read what the judge must also read:
http://sentencingcoun...ine_-_Crown_Court.pdf
Chris