G-KEST: First in 1997 out with Britannia B757 via Al Ain and Singapore to Auckland - really uncomfotable and tight seat pitch - knackered on arrival then back with Air NZ B747-400 via LA - much better with more room due seat pitch increase.
Second time this year with Air NZ out in a B747-400 via LA - OK with same seat pitch as before however the upholstery is getting tired and so less comfortable. Same airline back but B767 to Fiji then LA - both nice trips and fairly comfortable. LA -LHR in B747-400 same problems as on way out.
Most scheduled airlines will seem much noticeably better for pitch than the Britannia 757s, because (IIRC) they operated in charter configuration even for those flights.
Air New Zealand is beginning a programme of interior refurbishment, which will finally bring their cabins up to date. You don't say when you're thinking of going again, but it may be worth your while keeping in touch with what's going on there. Have a look at
this press release, which says amongst other things "The refit will begin in the middle of next year and work on the last aircraft is expected to be completed by early 2006." The intention seems to be to retain the current pitch, which is about 3 inches deeper than on airlines like BA, Qantas or Virgin Atlantic.
One other option you might want to look at, if you are slim, is Emirates. I do not say that in jest: They put 10 seats across in their 777s, but the seat pitch in economy is also a deeper one than usual. I wasn't particularly happy, being a reasonably-built person, but slim people might actually find this quite comfortable.
Do you have any absolute bar on upgrades of any sort? World Traveller Plus may be an option if you are prepared to spend a little bit extra. When BA have fare sales, you can often get W class at something like £100-200 extra per sector on top of the economy sale fare. That can be very good value. It's not business class by any means - you are still largely in an economy-type product - but the improvement in both physical comfort and also general ambience in the cabin makes the package very worthwhile at that sort of price. The downside is that BA metal only flies as far as Sydney.
On that topic, both Virgin (only to Australia and then only from early December) and Air New Zealand (in their new cabin) offer or will offer a comparable premium economy product.
Hope this helps.