No need for a coffin at all a body bag is fine , the funeral directors will not like it though, they buy coffins in for £30-£40 and sell them for £700 to £2000 or even more.
Did you know it is fully legal to just deliver the body to the back door of the crematorium and have it disposed of with no ceremony at all? No need for a hearse a van or estate car will do. Once the death certificate is signed and the permission for cremation completed then you can just collect the body and deliver it to the crematorium in a body bag. You can get body bags on ebay for £1.50 each . So a fully legal cremation can cost as little as £80 . Get someone on benefits to arrange the cremation and apply for a funeral grant of £1200 , that leaves over £1000 to have a good p!ss up in memory of the departed. I am actually thinking of buying a bulk pack of 50 body bags , if I sell 49 of them it will make enough to pay for my cremation.