why it's worth paying to get a proper outside tap fitted

You can get outside tap installation kits aimed at the DIY'er for around £15 like the one below:

Although they seem initially cheap, I believe they are a false economy.

Firstly, they use a self-cutting screw to tap into your existing pipework. Believe me these have a tendency to leak. I have been out to a couple of customers this year where a good seal was not achieved and resulted in a leak. Trouble is, you need a nice straight, perfectly round piece of pipe, and then to get the sealing gasket on nice and square. Unfortunately, if you don't get a good seal first time then you might end up cutting out a piece of pipe and replacing it.

Secondly, a hose clip is used to attach the flexible pipe to the valve. I certainly don't like the idea of relying on a hose clip in my home to protect me from a potential flood.

Thirdly, in my opinion these kits are not a long term solution. Eventually you will probably end up replacing it. If you are lucky it might very well last you for years, but you have to remember these kits are made as cheaply as possible. I have replaced a few over the years.

So do yourself a favour and get a proper outside tap fitted. When I fit an outside tap I use copper and brass pipework and fittings, and a lever operated isolation valve with a separate double-check valve. An installation that will give you peace of mind and last for a long time.