It's hard to do! Even if you're disciplined, maintaining the correct perch above the bombers from which you can break up attacks is difficult. I should have maintained a bit more altitude, since I usually found I was I only fast enough to follow an attacker through his attack, rather than intercept him before he got there.
This was also really not fun in a Spitfire Mk.VB in mid 1942! Normally I love the Spit, but it is not a good escort fighter. It doesn't help that at 20,000ft in August of 1942 puts the Spitfire at it's worst compared to its opponents. Unless you were one of the lucky few to have a Mk.IX that early, you really feel disadvantaged vs the Fw 190 in a Spitfire. I'm a solid 20 mph slower than them in the Mk.V, they have better acceleration and really just better everything except turn radius, which is not useful in the escort role. I flew this mission multiple times before I figured out just how careful I had to be about my positioning.
The design choices of the American fighters really start to make a lot of sense after this frustration. Lots of fuel, lots of speed, and lots of firing time. The Spit, especially at this time, has pretty short legs, and even on a fairly short flight into Belgium, it's got me keeping an eye on the fuel gauge. Ideally you need to be 20+ mph faster than what you are defending against so that the radius at which you can effectively intervene around the bomber formation is large enough to cover the formation. In the Spit, my positioning had to be perfect, and it rarely was anything close. The choice of the .50cal also really makes sense as the ideal weapon to defend your strategic bombing campaign. I've often wondered why the US fighters didn't go with the 20mm, like just about everyone else. Escort missions are exactly why. My .303cals were too light to drive off an attacker with a quick burst rather than chasing him all over the sky and losing position, and the 20mm Hispano, while arguably the best air-to-air weapon of the war, doesn't have near the firing time - especially with only 60 rounds per gun in the Mk.VB - to give you something to shoot with through repeated attacks. The .50cal is just heavy enough for the job (remember, you don't need to make the 190 disintegrate, you just need to make him think twice about sticking around for another pass at the bombers), it's got good effective range, which allows you to cover more sky around the bomber formation, most importantly, you can carry a ton of ammunition for it, which will keep you in the fight through multiple engagements on your way to the target.