Resignation,notice period and written offers

[Jettagti]

I work in retail management. It's not glamorous, but it pays the bills (just). I have been offered another job in a similar role which I would like to accept.
Firstly, should I wait until a written offer has arrived before resigning from my current role? I have been assured the job is mine
Secondly, my current employer states a weeks notice for every year of service up to a maximum of 12 weeks. I've been there 14 years so should offer 12 weeks notice. Can I be made to work this? I understand that not doing so is technically a breach of contract .I am about to have two weeks holiday, so was thinking of offering 6 weeks notice. That way my holiday stands, then a further 4 weeks working with them, giving them 6 weeks to cover my role but I will only be working for 4 weeks.

Any advice and opinions would be appreciated .