(I hate to beg, but: relevant friends in Oxford/London, please may I crash on someone's floor afterwards for the night, if it's not too much bother? The train connections I need to get all the way home in the evening are mostly absent, and it would be a massive help. I can compensate you!)
The protest is against the, quite frankly, evil cuts this government is making to Welfare for people with disabilities - among other things, stating that they have a target of reducing the people who are paid Disability Living Allowance (which is paid regardless of working status, and designed to reflect the additional costs to living associated with having a disability) by 20%. To stop paying Employment Maintainance Allowance after a year (because disabilities totally vanish when there's no money!). To remove mobility payments to people in care homes, meaning that many will become virtual prisioners - these payments are often used by care homes to, eg, fund an accesable minibus, and to provide wheelchairs. Cutting the respite care hours for families, including those where a child takes care of a parent.
None of these measures are going to solve the deficit. They are penny-pinching, and they are going completly against promises made by Cameron during his election campaining, and they are downright evil. They are going to kill people. (I'm not joking.)
You can read an article at The Guardian, here, which sums up the key points pretty neatly, and if you want more details on what specifically is affected, and how, you might like to read some of the posts on Where's The Benefit?, a blog set up in response. Or personal stories on how these cuts are going to have huge and horrible impacts on people on another related blog, One Month Before Heartbreak.
Here is the page with the information about Monday,. I'm nervous about going on my own (outside! in a big city! with strangers!), but to me, after having been following these blogs and reading about the planned cuts for months, I think I have to. This is important.