Not for the first time Dotty is basing her answers, in relation to working hours, upon the practices of her own employer, rather than on statute law.
As others have indicated, if your shift EXCEEDS 6 hours, you're entitled to a single (unpaid) 20 minute break, which can't be at the start or end of your shift. (If your shift is EXACTLY 6 hours you have no entitlement to any breaks). Your entitlement is neither 'pro rata' nor 'cumulative'. i.e. if you work a 12, 18 or 24 hour shift you're still only entitled to a single break of 20 minutes.
Your length of service has no bearing whatsoever upon your statutory entitlement. However a worker who is under 18 is entitled to a single (unpaid) break of 30 minutes if their shift exceeds 4½ hours.
There are some occupations where there is no automatic right to any rest break (e.g. running a railway station - I write from experience!) but you should get 'compensatory rest' instead.
It's all in Bednobs' link.
Chris