Selected Answer
Yea, you can do A LOT of things in Excel and this is certainly one of those things.
How you do it though depends on how you want the spreadsheet to look and function.
The simplest way to set this up is probably a math formula that calculates if the rented time is greater than 1 hour and then multiplies that amount by 10 plus 10 for the additional 10 pounds that the first hour costs.
Here is a formula that could get you started:
=IF(A1>1,A1*10+10,20)
This assumes that the number of hours is in cell A1 and that you charge a flat fee of 20 for anything that is 1 hour or less.
Once you have the total amount you can then do whatever you want with it.
If you need the cost per 30 minutes, you could do something like this:
=(B1/(A1*60))*30
B1 contains the first formula I included.