Is my employer breaking the law? Wales/nationwide

So I've recently started working for a large retail company that employs close to 100k employees across the UK. They are a UK based company whos complete operations are uk based although owned by a private American company. They also work closely with a major union in respect to the retail workplace that handles contract negotiations amongst many other aspects of the workplace that the union can influence.

My question's relate to the breaks offered in conjunction with the working time directive. The WTD states that any work period over 6 hours is entitled to a 20 minute uninterrupted break. So its come as a shock that my shifts are 7 hour 45 minutes long and im informed (by managment and company handbook) im only able to have 15 but can 'opt' to take an extra 15 to take it up to 30 minutes total. They arent splift shifts or anything other than a regular shift of start and finish. All of which is unpaid which is unsurprising but how are they able to do this legally between the company and the union? Also my contract is 30 hours which is covered if just taking the 15 minutes on my 4 days of shifts but if I opt in to take the extra 15 minutes daily ill be below my contracted hours each week. So does me opting in to the extra break then mean they are in breach of contract if im not going to be hitting my 30 hours a week?

I will be speaking to ACAS on friday as thats my next realistic opportunity to do so but im asking here to maybe be enlightened enough to not need to make that call.

Tldr. Employer offers 15 minute break on 7hours 45 minutes shift is this legal? Does me opting into an extra 15 minutes and taking me below my 30 hours contract make it a breach?