Charging two people two different prices would leave you open to an allegation of discrimination - be it sexual, racial, disability etc. It would be very difficult in court to prove that you were
not discriminating against one of the parties.
It is also illegal to show misleading prices*, so Buenchico's second paragraph (Advertising for �50 and attempting to sell for �1000000) is also incorrect.
Finally, there is no legislation to what happens to a price after the sale, but there is legislation to the price
before the sale - it must have been on sale for at least 28 days
in that store unless advertised differently (eg you will often see the term: This item was on sale in at least 6 of our stores nationwide).
B&Q are masters at this:
http://www.independent.co.uk/news/uk/home-news /you-could-rue-it-if-you-bq-it-1418239.html
* Consumer Protection Act 1987 Part lll