I am moving to London from September and was thinking of using Airbnb for the first 4-6 months before finding a more permanent place. But I find Airbnb to be more suitable for vacation and not so much for longer stays. Are there any services like Airbnb that allow me to find a room for longer stays?
There are numerous websites in that market, just search 'short term rooms London'
The specific landlord you deal with will matter more than the agent who you go through. This goes for your long term choice as well.
I would suggest airBnB for 1 to 3 months while you make some connections and get to know places.
There are several apartment-hotels that you can rent for long stays. Mostly these are for mobile workforce (e.g. consultants who move around EU and stay in each country for a couple months). Or you can find "serviced apartments". Both can be expensive (although discounts usually apply for long stays).
Both options are usually furnished, and comes equipped with everything you need (down to forks and knives). and for the apartment-hotels, you can pick those with or without kitchenettes. So you have some options.