that budget won't get you a lot of decent bike sadly - of any type, road, hybrid or MTB
do you have a friend you could borrow a bike from?? that way, you won't have to lay any dosh out and if you find that you like tri (and most do get hooked) then you can look at investing more funds towards a good bike.
if you can borrow a bike, make sure it's from someone of a similar height to you as if it's either too small or too big, it will be uncomfy
a decent bike for tri will cost around £500 minimum (new) although you could get a good deal for that amount on a 2nd hand one that would cost a lot more new. or - wait until autumn/winter when this year's stock is marked down to make space for next years
hope that helps and good luck