When I type in my postcode and house number into that OpenReach website, I get options, like:
Ultrafast Full Fibre Broadband - Not yet available
Gfast Fibre Broadband - Available now
Superfast Fibre Broadband - Available now
Standard Broadband - Available now
There is more technical info, including what speeds you're likely to get, at
https://www.broadbandchecker.btwholesale.com/
If you share a screenshot of this, with your address blanked out, we can advise what sort of service you're likely to get.
You've said G.fast is available; this suggests that you're close to the cabinet, in which case a standard FTTC service is likely to run at the maximum possible 80M down/20M up, or you could buy a marginally cheaper 40M/10M service. Either way will match or beat your current Virgin service.
Do all broadband providers charge for installation?
No. Most of them are free or a nominal charge like £10 "P&P" for the router. But of course, you're often bound into a typical 18-24 month contract, which is how they recoup the setup costs they have.
Some of them give cashback, either in the form of a prepaid Mastercard, or if you signup via a cashback site like Topcashback or Quidco.
I was further scared off by threats of charges of £65+ if I wasn't in the house when the engineer turned up.
It's standard. Normally an engineer visit isn't required for activating a service, but in your case with the line having been unused for "decades", you should definitely go with it. They may put a new master socket and/or faceplate on.
You should get an AM/PM appointment slot, and Openreach are fairly good at keeping to them. I'd take AM if possible, as it's less likely the engineer will have been delayed at another job.
So as of now, I don't know what to do, as I don't think I could even afford the switch and any extra charges that might entail.
I don't understand what you're saying. You could find your monthly charge goes down from £46.50 to around £25, which over 2 years would save you over £500 - so it's more a question of how you can afford *not* to do this.
Also this is a benchmark. Get an online quote from a low-end ISP like Vodafone, go back to Virgin and say "I'm going to Vodafone at £x per month unless you drastically reduce your pricing, at least match what you're offering new customers". The chances are that they'll do so, especially if it means they're getting you to sign a contract with Virgin. But if they don't, it's really not a big deal to switch to an Openreach-based service, and could save you huge money.