Given that megaships supposedly have "hyperdrives" rather than FSD's, I think we should be reviewing whatever we know about hyperdrives in Elite lore.
Discounting the supposedly "lost technology" Quirinium drives and other possibly non-canonical stuff, we have:
1. Ship transfer is supposedly via megaship. Thus, ship transfer speed gives us an estimate of hyperdrive speed. IIRC, it takes 61 hours to transfer to Colonia, about 22,000ly. This gives around 360ly/hr, which is a suspiciously convenient 6ly per minute.
2. The Gnosis can jump up to 500ly, once per week. If we assume that NPC's are refuelling the ship (or otherwise faffing around, maybe running surveys etc) during the week, at 6ly/min, it would take 1 hour and 23 minutes to actually jump that far (of course we actually know that it's relocated on the server tick, but at least this gives a timeframe for the imaginary jump). We can also infer that something limits the jump range of each ship.
3. Jacques Station jumped from the Bubble to Colonia in one jump (and was intending to go to Beagle Point). From this we can infer that the range-limiting factor is not absolute: there is no technical reason why a ship can't be built to jump right across the Galaxy (but the Gnosis wasn't built to do that). IIRC, there are non-dockable megaships capable of jumping thousands of lightyears too.
4. Jacques made the jump without refuelling, and took a whole station with him. From this we can infer that it was possible for him to carry enough fuel to jump a whole station that far (and actually further).
It is entirely possible that some megaships use a fuel with a higher energy density than liquid hydrogen, especially given that hydrogen fusion is a long way from 100% efficiency (only a tiny percentage of the mass of the hydrogen is converted to energy). Matter/antimatter annihilation is 100% efficient, but antimatter is diffcult to make, contain, and use (you tend to get hard gamma rays radiating in all directions). Earlier versions of Elite mention "military fuel", there is precedent for something sitting between hydrogen and antimatter. If it's used, I'm hoping we can still use hydrogen fusion to synthesise this with the right equipment onboard (probably multiple tons of hydrogen to make each ton of miltary fuel).