We've had a similar issue. You can solve this quite easily without spending a small fortune. Definitely speak to the local Building Control at the council and ask what is the minimum work which needs to be carried out in order to permanently keep open your planning permission - explaining that you will wish to commence work soon but you just don't have a set date yet. You carry out the work (in our case laying 1.5 m of drainage pipe), Building Control inspects and approves the work, and that you have commenced work. Your PP is now permanent and you will not need to renew, as work has commenced. Finishing the work is kind of open ended, commencement is not.
I suspect it really depends on what your PP involves but there should be a way to commence without major works. Ours was a conversion on a lIsted building, so the drainage was an easy commencement and was part of the approved PP. As Building Control have to sign off on any new work, they are the best ones to ask about this. I believe from memory it has to be 'new' work rather than repairs/renovations.
Here is a link that gives you simple(limited) information. See "Starting to build where Full Planning Permission or Reserved Matters has been granted" There's more information out there but this will get you started.
https://www.planningni.gov.uk/index/adv ... opment.htm