How easy it is will depend on how much student finance you're getting and if you qualify for the Imperial bursary (if your parents earn under a certain amount you automatically qualify). Overall I'd say you'd need a couple of thousands pounds on top of the basic student finance loans to be comfortable, but more than that if you want to go out a lot.
I didn't qualify for the bursary or any additional funding from student finance and I survived so you'll be fine as long as you're sensible! In the first year most of my student loan went on accommodation (I only had about £100 left and I didn't even have an en suite room
). Luckily I had some money saved up so I could still go out and have fun. I would spend around £10-20 a week on food, around £20 a month on transport and I'd have about £50 a month to go out and do things. After the first year you'll have to find your own place (unless you stay on as a hall senior) and I found it was much easier then. I chose to move outside of London where rent is much cheaper, but I have an hour commute everyday.
My advice is to keep an eye on your money and make sure you have enough money to cover rent and food. After that you can go out and enjoy yourself. If money is tight there are lots of free things to do as well so don't worry about getting bored. I love going to Hyde Park and the Natural History Museum, both of which are free and under a five minute walk from uni. I think it's also worth getting a student bank account because of the interest free overdraft. I don't know if they're doing this anymore but Santander offered a free railcard with theirs so you can get 30% of train tickets (great if you don't live near London!).
Imperial has a huge mix of people so you'll get people coming from very disadvantaged backgrounds and people who come from very rich households (although most people are somewhere in the middle!), so it may be worth making a budget for yourself based on how much money you know you have. Just assume you're going to spend more than you're planning on and don't worry if you don't stick to it! If you want to go out to parties as well then plan on spending a lot on alcohol because it's very expensive to have a drink out in London (I've seen £6 for a pint).
If you have any specific questions then feel free to ask me!