That's definitely a bummer.
While it might be legal, I think they are on thin ground unless you were looking at a section marked for business users.
I felt sure there was something about 'retail prices' being shown for non-business customers and 'retail prices' would include VAT.
In the past the same sort of thing annoyed me when PC magazines would review "10 machines under 800 pounds" but that was the ex-VAT price. Magazine was littered with ads and some were +VAT while others included it.
I just went onto the site and for the particular Apple iPhone I selected, there was some small print saying "Line Rental and monthly charges stated at the current rate of 15% VAT, this may change in line with Customs & Excise regulations." - I made the 15% bold as it clearly is out of date.
The front page of the site shows some prices and doesn't have any asterisk or small print about VAT. The Terms and Conditions PDF does not seem to mention VAT but has a link to a page where each network is mentioned and under Orange there are a couple of dozen T+C pages (depending on deals they have run).
As they say they operate through e-mail, and this was an online purchase, you might still have a 7 or 14 day "cooling off" period so may be able to either cancel the contract, or get them (mobile.co.uk AKA Carphone Warehouse) to pay the extra as they are at fault for not showing whether VAT is included.
This Government website says it is a legal requirement to show a VAT inclusive price with equal prominence to a VAT exclusive price for sites which have both consumer and business users. I think that should cover your case!
You could use
Distance Selling guidelines to get firm to 'put things right' (mobiles.co.uk website suggests a full refund may be possible)
There are some consumer questions on this What Consumer website - NB despite having a link and logo showing the phone number of consumerdirect.gov.uk this is not a government site - whatconsumer.co.uk is owned by a media firm.