Welcome to FlyerTalk
landersd.
As well as the articles linked above, there are many useful resources on this forum. In particular, see the Companion Voucher master thread here:
https://www.flyertalk.com/forum/brit...er-thread.html
There is a WIKI/FAQ at the top of the first page that you should expand and read.
Now, based on what you have said, I think you may find the voucher is less useful than you hoped. A lot will depend on when it was issued, but I suspect the answer to that will be unfavourable.
Firstly, the vouchers can only be used on flights operated by BA (colloquially "on BA metal"), so forget any thoughts of using it on Qatar etc.
It has to be used on a BA flight.
EDIT: I'd forgotten the recent change in this space, apologies. The vouchers can now be used on Iberia and Aer Lingus as well, but Qatar and other Oneworld partners are still not possible.
Secondly, if your voucher was issued
after September 1st 2021, then it will be one of the new style ones that come with Free BA Amex, and those vouchers are restricted to redemptions in economy class (World Traveller as BA calls it for long haul flights). These are widely regarded as being very poor value because as well as the Avios, you need to pay BA a substantial amount of Taxes, Fees and Charges ("TFC"). The bulk of these are often BA imposed surcharges, despite BA often (and incorrectly) calling them taxes. The net result is that you may find that the amount of cash you need to pay to cover TFC is not significantly less than the outright cash price to buy a ticket
Where these 2-4-1 vouchers do bring substantial value is used in the Business and First cabins, because even though the fees can be hefty, the amount is still often much less than an outright cash fare. However, for vouchers after Sep 2021, that is restricted to those earned on the Premium Amex.
You've suggested you are flexible on where you travel - in that case, I would suggest that you use the BA mobile app, as it has some useful search features for this scenario, with a tool called the "Reward Flight Finder"
You can find this under "Reward Flights" -> "Reward Flight Finder" and it lets you see availability across a broad range of options, e.g. you can search for all available seats in a given continent or country, for a given month. So you can quickly see what is available, to narrow down your options.
If you are booking an open jaw (i.e. returning from a different airport), you will need to telephone BA to do that, but the search tool will show you what is available so you could identify what seats you wanted ahead of making the call.