In no particular order:
Kippers.
A good pub* (preferably in the company of old friends).
Interstate in Covent Garden, so I can buy some new jeans. Been buying my jeans (as well as sundry satchels and undies) there for about 12 years or more. [Edit: Interstate closed down while we were in Roma! End of an era]
Vaguely reliable, functional postal services.
Cinema. There are plenty of cinemas here, but unlike in Paris say, it’s hard to see English-language films in versione originale. And I’m damned if I’ll watch a dubbed film, especially in a language I don’t understand very well. I detest dubbing. There is one cinema here that shows films in VO, but for some reason they had The Iron Lady on there for three long effin’ months. I adore the big screen, indeed it was central to my job for a decade or so, so this dearth of big screen action is a difficulty for me.
Simple brand products – soap, roll-on etc that’s not perfumed, not coloured, just kind.
Things I don’t miss:
Trashiness.
The sheer trashiness of Britain and British culture.
The unfailing uniformity of British shopping streets (mobile phone shops, Boots, Tesco Metro, generic coffee franchises etc).
The grotesque ubiquity of CCTV. I remember my feelings of shock and discomfort when I first became aware of CCTV cameras, such as outside a bar in Radford in Nottingham, c1992, where dealers congregated. Thanks to Blair and co, we’re all treated like potential now criminals in the UK. So much for valuing our freedoms. Never mind the Olympics factor.
The lack of lizards.
By no means a complete list. And is it prejudiced and classist? Who knows. Me ne frego.
* I don’t necessarily have a painful longing for British beer. As much as I love a pint of proper British ale, there’s no shortage of decent beer here in Italy, thanks to what I understand to be a fairly recent growth of artigianale (artisan, or traditional) beer production.
In Rome, we just need to go to Ma Che Sieta Venuti a Fa’ or Open Baladin, or other birrerie (beer bars), or specialist beer shops. We can even get great ales from the supermarket. Last year, the boyfriend of a friend launched a new beer in Italy, and after being unable to source it in the specialist shops, I spied it in our local supermarket. And very nice it is too: Mastri Birrai Umbri.
Bread, Cakes And Ale 
I find the postal service just fine … but there are times when I would kill for a kipper.
I think we are making some progress when a list about things you miss about home includes things you like about Rome (in a roundabout way). I miss the Vietnamese cafe on Old Street, cycling to work, and spending an afternoon/evening in the pub (with friends, old or new) without feeling like too much of a lush or outstaying my welcome. Baladin and Ma che siete.. are fun and the stout options are good but not wholly suitable for lurking at length.
I wouldn’t miss the dog poo and the incessant and X-rated swearing in the street by almost everyone. Nor the bad coffee (sometimes great, sometimes dishwater, no way to tell).
Apart from friends, I would miss my beloved marmalade (had to say that, obviously). I’m told it’s good with kippers, by the way.
I think your ‘things I miss’ list is reassuringly short. You must be enjoying Rome!
Ironic you mention poo, as I’ve found the poo in (our part of Rome) far, far worse than the poo in (our old part of south) London. And in Britain, you don’t have to contend so frequently with the dog poo + 35C plus heat as frequently. That’s particularly charming.