Hi Mozingod,
There are quite a few potential mandatory expenses you've not included here. I'll attempt to fill those in. There may well be others too, but I'll cover the immediately obvious ones.
As mentioned, Council tax is mandatory and will most likely need to be paid via you directly to the local authority. Sometimes, in rental costs, this is included so indirectly you've paid it. It's hard to put an exact figure on as it depends on the area and type of property and each local authority's pricing band/levels. In and around London, I'd estimate a figure of perhaps £150/month - could be quite a bit less or more of course.
Insurance - it's always wise to insure your belongings including anything of higher value.
Savings - How much would you intend to put away each month?
Emergency funds - for those unexpected costs.
I'd say those above are quite important mandatory costs each month in order to maintain as best as possible your chosen standard of living. The ones below will be more discretionary
TV license - could be grouped as more mandatory though. I think it's £160 for the year for a colour TV license.
Internet - this cost as mentioned I think is quite a large amount, however, depending on your usage it may be about right. If you are a more casual internet user, then a cheaper package will most likely suit.
There's other elements to which could be varied - have a look at this similar thread here on this forum, including the OP's view after a year I put forward 'relative' amounts!
http://talk.uk-yankee.com/index.php?topic=57943.0Cheers, DtM! West London & Slough UK!