As per my post above – you can only take your employer to an industrial tribunal if you have been employed by them for at least 2 years (there are a few exceptions to this).
If you sign and agree to the new terms, then you have agreed. If you don’t sign the new terms but nevertheless work under those new rules, after 3 months you will be considered to have accepted the new rules.
But if you do not accept the new rules (and refuse turn up 15 minutes early each day) your employer will have two main options, to continue your employment contract as is, or to dismiss you (they could come to some other arrangement with you).
If you do plan to refuse the new rules, I would advise that you put that in writing, otherwise they may claim that you are arriving 15 minutes early every day and have accepted the new terms.
You only have 3 months in which to bring a case before an industrial tribunal from the date that the new rules came into effect.
If you are considering taking a course of action that could lead to your loss of employment – I would strongly recommend you take advice from an organisation such as ACAS, rather than those on AB.
This is all very worrying, it appears that UK employers are treating their staff as serfs.