My 2p worth, I've taken a bike twice on easyjet. Go to a bike shop and ask for a cardboard bike box, the kind that bikes are delivered in. Take your wheels off, twist your handlebars round. Find something to wedge between your dropouts (fork ends and back wheel mounts). Get lots of bubblewrap. Also pipe lagging from a DIY shop. You may want to unship your rear mech to prevent it from damage.
You haven't said roadie or mtb? Road bikes are obviously a bit more fragile.
I did borrow a soft bike bag for my second trip, no real advantages apart from a more compact pack and big handles.
I've seen people bring fully assembled bikes to check-in by the way, as far as the handlers are concerned it's probably easier to deal with a whole bike. Holds are pressurised so there's no need to let down your tires.
Check with your airline. It's not to much of a faff, have a think what you'll do with the box / bike at the other end of the journey too.
The only small thing I would recommend is to fly direct if possible (obviously not possible if your destination is Kabul or Pyongyang) .
This might mean having to traipse over to London, Birmingham or Manchester rather than flying from your local airport (Bristol?).
Most luggage gets lost in transit. Ordinary suitcases can usually whiz through robotic baggage systems. A bike box might need manual intervention which increases the potential for delay and theft.
Also check possible excess baggage rates as you might have to pay to include a 2nd item of baggage (unless you are in an airline which charges for *all* baggage). Some airlines will upsell you to the next cabin (with higher baggage allowance) for not much more than the cost of a 2nd bag.
Cheers peoples , i will do some more digging for info. , as far as airports go Heathrow is easiest but also Brum is fine , Bristol although the nearest can be a pain to get to at certain times and all those " Vicky Pollard " accents , aaaagh ! ha ha ha