Unfortunately there is nowhere that you can read One Foot in the Grave for free, as if there was, then the author would not be able to make a living. People buying their books is how authors earn their money.
However I have a few tips that will let you buy the books for a cheap price.
The first place that I go when I am looking for a book to read is a local charity book shop.
Usually the books are organised in alphabetical order, so it will be easy for you to pop in every now and again to check if they have the title that you want.
Books from charity shops can start from as little as about 50p, and it's very unlikely you will pay more than £3 for one.
The next place to check is eBay.
People often put used books on here and sell them for a couple of pounds. Although make sure you don't get charged too much for postage!
If you want the book new, then try Amazon.com
At the moment Amazon is selling the books separately for about £6, or you can get the whole set for £17.
If you have a Kindle then it is even cheaper, you could read it for only £5.49, but make sure that you have a device you can read this on before you buy it.
You could also try well known books shops on the high-street, but when buying from these kind of retailers, you are likely to pay top price for each book... so I would advise buying online.