Not an expert on UK training but I will have a go.
1.after going solo which my instructor told me it will be in 2-3 hours, can
I simply hire and build hours or still need to hire at the price of instruction
rate even when the instructor is not with me ?
The charge rate will be dependant on how your school charges. But after solo, the lessons keep going, there is plently more to learn and you are going to need all your minimum hours and perhaps a few more. Hour building is not really something you do until well into a CPL
2. once I pass my license and can hire airplanes, most places qoute on
hourly cost , so considering a £160 per hour for a c172 , if hired for 24 hours
the cost will be significant while you may be only flying for 1/2 hours. so for
instance flying for leisure , i want to go to france which takes me 1 hour to
get there and might be there for a day before returning so in total 2 hours of
flying but 2 days . so do i need to pay £160 * 48 or *2 ?
You usually only pay for the time flying however there may be minimum hours that need to be flown. Usually a couple hours a day. But this can change from place to place. A school that primarily teaches during the week may be happy for you to have an airplane all weekend and only fly 2 hours, you need to check out different places. Also you may find places are more willingly to give you an underutilised aitrcraft for longer (perhaps the old tatty one)
Remember when comparing prices one hour is not equivalent to another, tacho hours, hobbs hours and start to stop hours may all be different.
3.when you land on other airports, do you have to give them advance
notifications or do you just enter the space and request landing ?
I don't know about EASA land, but not usually where I fly, unless private strips or military.
4.if anything goes wrong with the plane while you are away from the rental base,
whose resposibility is it to get it back? for instance technical failure that
costs 2k , i shouldn't be responsibile , should i?
You need to check this when you rent. Kinda like a rental car, you need to know what insurance there is and who is responsible for what
5.when doing solo , can you take a friend with you or should be on
your own ?
On you own. When you get a PPl and start taking passenger you will realise why they would be dangerous to have when training solo