If you're doing a buy to let make sure you do proper calculations before hand and don't get sucked in to the idea of owning an investment property, unless you can afford it if things go wrong or unexpected costs come up!
Void periods can be a pain in the arse, it all depends on the desirability of the property, the market and your price point, but I'd factor in for a void period of at least a month inbetween lets, of course it might so happen you line someone up before your last tenant has moved out but this is likely to be a bonus rather than a standard.
Factor in service charges, maintainence costs (eg boiler, washing machine, leaks, etc) and importantly any the costs of an estate agent. Whilst you may be able to find tenants through friends or through loot, certainly in London, if you want to attract higher rents (eg. investment bankers who probably don't bother with loot) you'll be going through an estate agent. Foxtons charge 11%+VAT for their unmanaged service (probably at the high end) and most charge this up front so you effectively loose your first month's rent and a bit.
If you're using an estate agent you can probably do the inventory and contract yourself which will probably avoid their own extortionate fees. Standard shorthold contracts can be found on the net easily enough (£10 or something like that), an inventory anyone can do with a pen, paper and a bit of time.
Also, if you're worried your tenants might mess you about there are insurance policies out there which will cover any missed rent - useful but obviously come at a premium, depends how much your trust your tenants