Having had exactly this, being non rated with thousands of hours on Boeings. I would not have done it any other way. Getting in a month earlier rather then later would make a difference of at least 30 places on the seniority list at the moment! LH would probably have given a slightly easier lifestyle initially, but being already over 1/3 down the fleet seniority list I already have control about my working lines.
I had the same as Jumbo and had absolutely no regrets. I joined on the 757/767 (from a 757 operator) while almost all my former company colleagues got to go to the 744. I was initially a bit cheesed off and thought about delaying my joining date (it was an option then) in order to join them but, in the end, was convinced by a BA mate to just get in as quickly as possible. I'm extremely glad I did and very pleased that I didn't go the Jumbo straight away. While I went whizzing up the 757/767 seniority list and got to, very quickly, a position where I could get decent lines of work, my mates on the 744 were stuck at the bottom of the list with very few people entering below them but a lot, transferring from other fleets, sliding in above them.
BA is all about lifestyle and that means seniority. Being more senior on a junior fleet is much better than the other way round and gives you many more options for controlling your work and days off. Don't be in a huge rush to jump onto an aircraft just because it's bigger than another one in the Company armoury; they're all the same once you close the flight deck door and nobody gives a damn that you fly a BA widebody....that it's important is just your over-inflated ego speaking.
End of the day, do what my mate said; get in as soon as you can and if you're offered shorthaul, grab it and enjoy the control of your life that you'll have compared to the bloke/blokess on your course who went to the 777.