If the court regards it as 'minor damage' (equivalent to breaking a small window) the 'starting point' sentence, from which the magistrates must work up or down, is a fine equal to your weekly income (after deduction of tax and National Insurance). Only a fine (of up to 175% of your income) or a conditional discharge can be imposed; the court can't issue a Community Order.
If the court regards it as 'moderate damage' (equivalent to breaking a shop's plate glass window) the 'starting point' sentence is a low level Community Order (perhaps involving 40 to 80 hours of unpaid work). The lowest sentence which could be passed would be a fine of 125% of your weekly income. The highest sentence available would be a medium level Community Order (possibly involving up to 150 hours unpaid work).
Notes:
A. The fines above assume that you're earning at least £120 per week after deductions. If you're not, the system will assume that you're earning £120 per week (even if you're only getting JSA).
B. The decision to move the sentence up or down from the 'starting point' is based upon examining 'harm' (i.e. the value of the damage done and/or the psychological impact on the victim) and 'culpability' (i.e. whether the damage was the result of little more than carelessness or a deliberate malicious act). If they're both 'low' the sentence is likely to be close to the minimum available. If they're both 'high' the sentence will be close to the maximum available.
C. Community Order can involve things other than unpaid work, such as curfews or requirements to attend courses (e.g. 'anger management').
D. Whatever happens (with damage of only £300) you can't be sent to prison.