Maybe patience is the key - you say you don't have any but maybe now is a good time to learn some.
OK so you have a puppy that needs time and money spent on him/her, rent, phone, etc.
Start yourself a budget sheet list all of you outgoings and then list that against what you have coming in.
You have to look at where you can reduce your current outgoings to accomodate your new outgoings.
Remember even if someone knocked on your door today with a complete tank set up (tank, stand, substrate, filter, heater, plants, etc and a voucher for fish for when your filter is cycled) you would still have to cover food, meds, etc not to mention the electric to run your tank.
You'll also need to have patience to wait while your filter cycles before you add any fish - that will take a lot of patience.
Maybe now is a good time for you to spend reading up about this hobby you want to begin, reasearch different fish you might want to keep, think about adult size, habitat, compatibility, etc
Put a little money aside each week ito the "fish fund".
£150 is less than min wage (based on my assuming you work full time) so you want to ask about that. If you're not working full time and aren't at college then maybe you need to look at a second income for the fish fund?
Fish keeping isn't a one off outlay - it's an ongoing money pit LOL - 'fraid to say but 'tis true.