My thoughts behind mentioning own business was to see if it prompts gardening leave.
One thing to add - you don't say whether gardening leave is a good thing or bad thing ?
Answer to this determines approach.
Do you want gardening leave ? - gives you time to sort out setting up business, vat, accountant, first contract etc. etc. and get paid for it by old firm - but you don't want them to apply the restrictions - then come up with another excuse to get gardening leave - I can think of joining a competitor - or considering that as a starter for 10 - might only apply restrictions to competitors not an organisation that doesn't even exist. When setting company up (assuming you haven't done this already) means you can specify the type of work (i.e Consulting as opposed to Employment agency services etc)
If you don't want gardening leave but to get out asap - then a different strategy could be the emotional one - your hamster dies, burn out, cant go on or one that suits your life style and believable - fallen in love with 18 year old Thai girl and need all the energy you can muster I think BEP used once :-)
As BEP says bite your tongue and do what's right for you.
Whichever is the path for you good luck !