It's not as simple as that mate. First of all, it's very difficult to introduce immediate changes because businesses need time to program their computer systems to reflect the new rules.
But let's put that aside and say Labour could introduce some changes that would apply tomorrow. They'd get a 1-time bit of extra cash but that would be it. After that, people would adjust their finances to avoid paying. I am no tax consultant but you're a business owner and you decide to incorporate your business in the Cayman islands and make a trivial profit in the UK, paying yourself buttons. For example. Or they just fuck off out of the country never to come back.
The point is, if you are not on PAYE and you are seriously loaded, you have all sorts of possibilities to exploit loopholes.
If it was as easy as you suggest to clamp down on tax avoidance, governments would have done it decades ago. Labour never managed it under Blair or Brown.