It's not a problem

Firstly if you want to use Etihad, there's no benefit in booking from here or there - as their site will have a uniform costing (Not like you're booking from netflights for example)

Now you probably know this, but don't forget Etihad, along with many others, have a matrix system where you can enter dates for your flights, put in that you are flexible - and it will bring up different prices for different dates. Some would say it's more fun experimenting with this than playing Space Invaders on the Etihad games in flight entertainment (Grief that was a boring game this afternoon).

Seriously though, regarding your booking this, I did something similar last July. I phoned Etihad customer services afterwards to ask what they wanted, and they told me all I had to do was go to the Etihad service desk at Manchester, present my card, give my wife to be's booking reference and authorise it there and then.

Now for me this was ok, as I had to be at Manchester airport that morning anyway (I was flying from UK, she was flying from where she had just finished her contract, and we were meeting in Abu Dhabi).

On top of this I scanned my credit card and e-mailed her this, for her to print out, but she didn't need it in the end.

Hope this helps, but Etihad are extremely approachable, so maybe if you give them a call and explain it, they can say what you need to do (And you'll get a name who you speak to)

As for an alternative to Etihad, well presumably your wife and children will be best flying to Manchester rather than a London airport, so Etihad really will be not only the best (And IMO they are fantastic and up there with the best anyway) but cheapest.
Maybe worth checking out Emirates as well mind.

For a suggestion, check your flight dates on netflights.com. These are owned by Thomas Cook, and unlike say Expedia (A dead loss IMO) they will usually find a much wider range of reasonable flights. They will also find the "best" flights for time of flight - they will bring up the slightly more expensive ones with say a 2 hour wait in transit, instead of ones a bit cheaper but involving 30 hour layovers. You definitely don't want those!

Edit don't forget if you're registered with one, to go through topcashback or quidco, and you can get a few squid back.