You only have an automatic entitlement to a pay rise if
(a) your contract states that you do ; OR
(b) you work within a pay structure defined by statute and the relevant review body decides that you should have a pay rise (e.g. teachers) ; OR
(c) you have a similar pay structure which guarantees incremental pay rises up to specified ceiling (e.g. local authority workers) ; OR
(d) there's an agreement between your employer and a trade union (representing you, or the majority of your co-workers) regarding automatic pay rises ; OR
(e) your pay would fall below the National Minimum Wage if you weren't given a rise.
If none of the foregoing apply you're NEVER entitled to receive a pay rise. It's solely a matter between you (or a union with collective bargaining rights) and your employer.
Every employee's contract is separate so there's nothing to prevent your employer from paying your colleagues, doing exactly the same job as you, far more or far less than you get. There are, however, a few exceptions to that statement. Most of them (such as discrimination on the grounds of race or gender) probably don't apply in your situation but there is a regulation which makes it unlawful to discriminate between full-time and part-time employees. So if your full-time colleagues receive a pay rise it's likely that you're also entitled to one.
It's up to you as to whether you approach your employer individually or together with your colleagues but, either way, the old maxim of 'if you don't ask you don't get' probably applies.
Chris