The loan was for two years, with interest payments of £150 (so, £75/year) on a total of £450. That works out at about a 17% AER. On other words, you'd have been about as well off to get the first credit card offer that came through your door, buy the phone outright, and pay back the money at the same rate. You'd have been a lot better off if you could afford to pay back £50 on your credit card bill every money. A quick search tells me that the Sainsbury's credit card has a 7.8% APR, so if you got one of these, you'd be a lot better off to buy the phone on the card, and then paying back as much as you could afford.
If you're in a situation where £450 is an unaffordable expense, I'd imagine that you already have a credit card that you pay off every money, so you postpone paying for your regular expenses by 14-45 days, in which case just buying the phone on the card you already have would be cheaper and no more effort.
And it sounds like you actually got a comparatively good deal on your phone. Most 'subsidised' phones are equivalent to a loan with an APR of 20-50%. I'd love to see the regulator say that phone companies had to sell phones at the same price whether you had a contract or not, but could include a loan for phone purchasing with the contract as long as they stated the terms with the same detail required of other lenders.