Although your MOT is still valid, you're knowingly driving an unroadworthy vehicle, etc - which is an offence without any regard to MOT status. You've been told about the defect meaning the car isn't up to standard required to be on the road, blah blah blah. I'm not sure how BiB/your insurance/His Honour would interpret it, but it likely wouldn't be in your favour.
Since braking efficiency has passed, you could squeak an MOT pass before you fix it - with a bit of pressure either side of the kink, does it straighten out? If so, I'd be tempted to splint it straight (splint on the OUTSIDE of the bend so it doesn't dig in and cause a rupture) and wrap with tape until your new hoses arrive and are fitted. You'd get an MOT pass and avoid the grey area should you be involved in an accident (your fault or otherwise)
A kinked brake pipe has usually collapsed internally, and isn't in the least bit safe. With the standard dual circuit systems, you're not going to lose all braking if it fully bursts, neither are you going to kill and maim a school full of angelic little children just about to win the nobel prize etc - but you also aren't going to be stopping as quickly as you could, or in a perfectly straight line.
Legals don't generally know/understand that, though - any time anybody hears "brake pipe split" they imagine no brakes and a fireball careering through the classroom at 120mph, mowing kiddies down like skittles. Dodge the pass with a cover up job to cover yourself, and then replace the pipes. Plus you get your free retest done in time to have it a cursory glance at the pipe and a nod to get on your way, vs a full MOT again.
The OSF flexy is too short on the goodridge hose kit, so I relocated my solid brake pipe to shift the joint, giving mine room to flex to full suspension travel on full lock as it annoyed me (you never know when you're going to be at full lock whilst flying over a humpback bridge...) The NSF flexy is longer - two NSFs would be better in the kit, but eh...
