For the activites you listed, the i7's power is certainly wasted, infact an i5 is a bit excessive. Space is a personal matter, you need to work out your storage needss.
More power......... Sorry, turned into Jeremy Clarkson for a minute then. Go for the i5 and pocket the change. It's still a shiny new mac
I have the 13" i7 with 256GB SSD, and I can happily say that you don't need the i7. The i5 is easily enough for 90% of people, with most of the apparent speed coming from the SSD rather than the CPU. Mine came to £1300, just under, with the upgrades and AppleCare extension FYI, so if you do decide to go for the 256GB SSD then your estimate of £1200 is about right. It really comes down to how much space you need. I felt 128GB would restrict me a bit too much, as I have had 80/120GB HD laptops in the past that I filled fast.
Locking it down? Nope. Apple do it in store with their custom MagSafe chargers and alarms stuck to the machine. The lack of a Kensington slot does hamper things - but then Kensington locks take about 2 seconds to open with a train ticket...
i have a macbook air 13 with the i5 and the 256gb upgrade. It isn't restricted as i use it primarily for the same reasons you intend to. If you are after more power wouldn't a 13 inch Macbook Pro be a better choice ? or waiting until the refresh comes out? But then you are losing the point of having something so portable. I leave all the big boy encoding and gaming to my tower waiting at home !