I'm not in Portsmouth, but I do have Sky Broadband.
It's good, & stupidly cheap.
But, as it's ADSL, then it does depend on the quality of your BT line, & how far you are from the exchange.
The further you are from the exchange = the higher the attenuation = the lower your possible max. speed.
I am "approximately 1.48km from the exchange (straight line distance)" & get 8-9mbps on the Sky BB Max ("up to 16mbps") service.
You can get a rough guess of your line distance here:
http://www.dslzoneuk.net/distance.php
(although of course the phoneline won't actually be in a perfect straight line)
Also, as mentioned above, speed can also depend on line quality. Quality can be affected by the line itself, the exchange, & also your own phone wiring (e.g. extensions etc. can introduce noise & reduce your noisemargin).
Has your exchange been unbundled by Sky/Easynet?
You can check here -
http://www.samknows.com/broadband/search.php
If it has, then you should be able to go for anything right up to the "Max" service ("up to" 16mbps).
If your exchange doesn't have Sky/Easynet LLU, then you'd only be able to go for the "Connect" service, which is a re-sold ADSL Max service (up to 8mbps). Still cheap, but not as cheap as the unbundled services.
I'd have a look around
www.thinkbroadband.com ,
www.skyuser.co.uk &
www.ispreview.co.uk for more info on Sky Broadband, & reviews etc.