What's the point?

I have always wondered why shops sell goods for (say) £5.99 instead of a nice round whole number of pounds.  The reason of course is to make you think that the item is cheaper because it is only £5 something instead of £6. I suppose I can live with that as long as I can get one penny change when paying in cash and am only charged the exact 99p if paying by card. However, two things get me thinking.
First, how much longer is the one penny coin going to be legal tender?  I am sure that it costs more to produce and circulate than it is worth and its days must be numbered. But I also guess that, even if the Bank of England decides to dispense with the one penny coin, retailers and the British Chamber of Commerce will try to delay its implementation as long as possible because it will make their products look more expensive. They cannot still price goods as £x.99 as you will not be able to have a 1p change and that would be illegal trading. I seem to recall that, when some continental countries had large denomination currencies where they couldn't give exact change, they would give you a sweet to make up for it.
My second thought, however, concerns that ridiculous point 9 of a penny on all prices at petrol stations (such as 123.9p per litre). If you buy 5 litres the total cost should be £6.195 = six pounds and 19 and a half pence. However, there is no such currency as a halfpenny now so it is highly likely that the machine will round it up to £6.20.  If the petrol station dispenses (say) 40 litres per car and there are (say) 200 cars every 24 hours that makes a total of 8000 litres. If every 5 litres makes the garage 1 halfpenny that equals 1600 halfpennies which is £8.  Multiply that across the country and that's an awful lot of extra money - quite a bit of which will go to the Government in fuel duty.