depends how good you are.
there is a learning curve, though, in terms of how things are done here, versus the UK.
More valuable here than just having a good eye is your connections- do you have reliable, honest, reasonably priced plasters, floor refinishers, plumbers, electricians, and so on?
Do you know the good places to find lighting, plumbing fixtures, and furniture?
Do you have relationships with good antique dealers, who tend to procure stuff directly from families with recently deceased elders, and who often have their own relationships with furniture refinishers, upholsterers, and so on?
Obviously, if you move here cold, you arent going to have any of those things.
Its going to take you a couple of jobs to start to accumulate those connections, and speaking spanish is pretty necessary.
Your best bet is going to be to find a great, honest, quality contractor to partner with- and they are rare.
There are wonderful things here- antiques that are really great, and by world standards, pretty cheap. If you really do your footwork, spending a lot of time going around to different neighborhoods, you can find amazing things, and arbitrage of antiques is a real possibility- If you are quick and decisive, there are great deals, even at the Pulgas.
But its not quite the even playing field that you might be used to, in terms of being able to primarily sell your eye and design skills- you cant just call up any glazier, or tile guy, and expect quality work, and poor quality will directly reflect on your reputation and your income.