Your £80 budget has just gone out of the window!
From a safety point of view I wouldn't be attempting Pen Y Fan in anything approaching winter conditions without wearing a decent pair of 3-4 season full leather boots and having ice spikes and flexible crampons in the rucksack to cover changing weather conditions.
A quick google of 3-4 season boots will point you to plenty of reviews. Price range £170 - £210.
I have a pair of Altberg Mallerstang for use in such circumstances
http://www.altberg.co.uk/product/the-mallerstang-boot-mto/For Ice Spikes there are currently 3 main choices - Khatoola Ice Spikes, Hillsound Trail Crampons or Grivel Ran Antislip. Price range £27 - £55.
Recent thread discussing ice spikes
http://www.walkingforum.co.uk/index.php?topic=35630.0For flexible crampons look at Grivel G10 or Grivel Monte Rosa but there are others. Price range £70 - £90
To get the best value for your money have a look at buying a 3 season leather boot which can take a flexible crampon for short periods of time (occasional use only but not confirmed by the manufacturer) so they can be used year round. Have a look at the Altberg Tethera or Altberg Nordkapp.
http://www.altberg.co.uk/product/the-tethera-mens-boot-mto/http://www.altberg.co.uk/product/factory-stock-nordkapp/Most of this winter gear can be picked up secondhand far cheaper on eBay or Gumtree but winter is not the best time to be buying winter gear as demand is at it's highest and so are the prices people are prepared to pay.
To top that lot off a decent pair of Gore-Tex or eVent gaiters will help protect and prolong the life of your boots.
Here's a previous thread discussing Pen Y Fan in winter conditions
http://www.walkingforum.co.uk/index.php?topic=33667.0You might want to consider sending a PM to some of the participants asking for their advice and recommendations.