There is a whole world of points forums out there! For the UK, headforpoints.co.uk is a good starting point but there are many other blogs etc. TBH it’s quite a lot of effort in the UK to get stuff for free.
Specifically with BA/Iberia/Avios, availability of redemption seats in the premium cabins isn’t great. BA only guarantee 2 business class seats (and no first seats) will be available for redemption on each flight and release those seats 355 days out; any further availability is a lottery. So there are some tricks to making the most of these schemes, but you need to have a plan (spend on credit cards etc to collect points by certain dates and some flexibility on when and where you fly to and from) and you need to be pretty dedicated to it. For upgrades, you need to have bought a certain fare bucket ticket and then have the next class redemption seat become available for you to be able to book the upgrade. Therefore you need to be content with the class of travel you have booked, in case no availability comes up. The alternative approach is to collect enough points for a return redemption, possibly using the Amex 2-4-1 voucher scheme. You can then plan to book business class seats a year before the departure (when BA guarantee 2 seats will be released).
We have happily used Avios for short-haul flights (within Europe and USA) and recently for a redemption to South Africa and I am happy with the scheme because I understand its limitations. We also fly quite a lot. If you spend lots of effort collecting with certain partners (i.e. shop in Tesco and Shell when you would prefer to shop at Waitrose) and then find you can’t get the flights you wanted you will be pretty miffed!
The alternative to the points game is to consider “ex-EU”. Direct long-haul flights from London cost more than connecting somewhere; similarly starting your journey outside of the UK can cut costs considerably. If you are willing to buy a separate flight to Dublin, flying business from Dublin direct to the USA can be much cheaper than from London and you get Preclearance which means you pass immigration in Dublin (can save a ton of time). (Just make sure that, if you fly AA you get the A330 to PHL not the 757 to JFK!!)
Honestly, if you don’t fly much then I would encourage you to get a cashback credit card instead, have a night in Dublin or Amsterdam before the long-haul and just pay for business class seats. IMO of course!