Dora,
If you think you'll have the knack for it then definitely have a crack. Before signing up for the course, you would be crazy not to invest a few evenings seeing if it was for you, by playing about with HTML. Then play with a bit of CSS to change some colours, spacing, make a table pretty, make some rounded box corners etc. Then try to do recreate some of the graphics you see on a simple web page you like. Starting from scratch you're going to find you hit the buffers every now and again with stuff simply not working. BUT put these times aside and judge from the rest whether you have aptitude for it. If you see the potential for the building blocks the online course introduces you, start to problem solve the margin/border/padding to get the layout you want etc and feel excited by the possibilities then great. If you don't have aptitude then forget it, since there are plenty of geeky 16 year olds who do, and it's not nice to compete.
Ok, now look at the course. To be useful in a team developing a site you really need to have a good basic grasp of not just HTML and CSS, but something a step up: Javascript and probably standard components like those from JQuery. If you can get objects to fade, react to mouse clicks and use and customise say a tree or pagination, you're useful. If your course doesn't cover javascript to this sort of level then I think you'd be struggling to be of use. More than likely basic PHP is necessary too. I'd guess these jobs are the ones around £20k previously mentioned, although I'm not familiar with this end of the market.
Of course the mid-level web developer, at perhaps £30-35k, with maybe 3-5 years experience, will be versed in a bit of linux administration to get say apache and varnish working. They'd probably know how to administer a web framework like Wordpress and write lots of PHP to customise pages, if not write a Drupal or Django app. And they'd have a reasonable knowledge of interfacing with other server things, like bash scripting, AJAX, XML/JSON - this could be a long list and really only comes with experience. And be good enough at Javascript to hack badly written components to get them to work.
You shouldn't need to worry about these latter things, but I want to persuade you that a beginners course in just HTML and CSS is not really enough to get you work or even design your friend's personal website for them, unless they want it to be incredibly basic, static, 1998 style.
You could usefully go to meet developers on a 'hack day', which occur in big cities occasionally, to see if you can get on with them.
Once you're in the industry, then I would have thought your relative maturity and skills you've already honed will be of great advantage. There are English grads in the industry who add a lot that the through-and-through geeks miss. And eye for detail, managing people, projects, clients, even sales, is in short supply in this field. If you can do these sorts of things whilst having a good grasp of technical issues then you can look at £40-£50k. Or if you genuinely to the technical side well and become in demand, then in 6-7 years good progress and wide experience, you could be on that too.
I hope this is of help, and wish you all the best.