Your reply reads like you are trying to justify your position as the injured party. I'm not trying to be argumentative, just pointing out the obvious. There was absolutely no mention of your parent in the fist post; it's all you saying I, I, I. "I am getting rid of..." "unaffordable for me..." "I am going to get..." "I have chosen..." "I have an existing Arial...." "I stupidly cut..." "I would never need to use it again." etc etc.
Even if your parent asked you to find the cheapest installer, it was you who came on this forum asking for advice so you're the one responsible for feeding that advice back so your parent could make decision based on our collective experience. Looking back I can see I advised you precisely that this could be done as a DIY job and I detailed how. You had the option but you've now said your parent wanted someone in for insurance purposes. Then you start moaning that it's not the price you expect. Well guess what, insurance costs money, running a van costs money, paying wages and employment tax and insurance and corporation tax and running web sites and paying for phones and premises and, and, and, all costs money. Supplying and backing up a warranty all costs money too. You've not said how long the guy was on site, but a meter-aligned aerial install for £166 is on the cheaper end of the install price spectrum. Making up your own price expectations based on what you pay for DIY gear just isn't commercially sound or sensible.
I said it before and I'll say it again, the predictions were made and you (and your parent) made your choice. Everything that followed after that choice is your responsibility. Don't moan when you choose to go against the advice of people you've asked for help.