Under the Working Time Regulations you are entitled to at least 5.6 weeks' paid leave per year (which works out as 28 days for those working 5 days per week).
The employer may count bank holidays against this entitlement.
Part-time workers are entitled to equivalent paid leave on a pro rata basis. However, bear in mind if a part-time worker does not normally work Mondays then when bank holidays occur s/he will not automatically be entitled to a paid day off.
The arrangements for taking this leave are sometimes specified in the contract of employment and sometimes in the company 'rule book'.
Where there are no express (e.g. written) contractual arrangements for taking leave the rule is that you must give notice of at least twice the length of the holiday requested. So if you wanted 2 weeks' leave, you would have to give 4 weeks' notice.
Unfortunately the employer may refuse your application on certain grounds, such as pressure of business. If this happens, think about whether the employer is treating you less well than others (i.e. is discriminating against you, and/or is breaching the implied term of mutual trust and confidence between you).
It works the other way too. If the employer wants you to take 2 weeks' leave unexpectedly, 4 weeks' notice must be given (unless there is a relevant agreement – such as a clause in your contract – specifying that less notice can be given).
If the employer does not want you to take leave at a particular time, say for 2 weeks early in the year because of a big order, they must give the equivalent period of notice, i.e. in this case 2 weeks.
Of course, in all cases, the employee and employer may agree to relax or vary these rules.
There is a helpful and free leaflet from the TUC Take a Break available on the TUC website.