The law entitles employees to reasonable time off work to undertake public duties, which include, amongst others:
There is no obligation on the employer to pay for this time off.
The above list does not include standing for election to a local council, so you are not eligible for statutory time off for this. However, with the exception of certain senior positions in public service, your employer cannot prevent you from engagement in political activities in your own time.
Also, while your employer is very unlikely to permit you to campaign at work, it is not permitted to treat you less favourably than others on the basis of the fact that you hold a political belief.