Hi and welcome to both of you :)
Adopting mixed race children completely depends on the attitude of the LA, the particular children waiting and your own skills/attitude I think. I know some white adopters have indeed adopted mixed race children, so some LA's are happy for it to happen with the right match. However others are not so keen. You would have to check and guage the reaction of each LA you spoke with. Of course it depends which LA you choose as some hae many more mixed race children than others. Bear in mind though that you don't have to adopt a child from the LA you go with. If in say 3 months you can't fnd a suitable child in your assessing LA, you can adopt a child from any other council, so even if all the LA's say you can only adopt white children, and they happen to have few of them, it doesn't mean you would never find a child, you just look further afield.
If you are interested in adopting mixed race children and the LA are okay with this, expect them to question you carefully...they need to see you are prepared and educated about what it means to be a transracial family.
Siblings bedrooms depends on the LA, but generally different sex siblings always need a room each. With same sex siblings, most places would prefer you had a spare room, even if they are starting out sharing their room. Siblings can suddenly start wanting their own space after all, or start fighting etc...there might be a number of reasons they might need to stop sharing and if you have another room you don't have to move house to accomodate this. Some LA's will always want a room each for all siblings.
LA's and VA's...all children are taken into care by LA's. If you go with an LA, they will always try and match you with one of their 'own' children first (this is called an 'in-house' match) before they will let you look elsewhere, to other LA's, exchange days or waiting child magazines. Mant LA's will as I said lock you in for the first 3 months after approval and only consider their own children. However as VA's (which are independent/private) do not have children in their care, with a VA you can look to all LA's immediately for a match, immediately go to exchange days, read the waiting child magazines and go on the adoption register etc. Because LA's will always try and match their children with in house adopters first, they will only look to VA's after looking through all their in house adopters, unless their are special circumstances such as the child needing a home a very long way from their birth family.
A few years ago, when there were fewer young children waiting adoption, VA's tended to specialise much more in placing siblings, children with special needs and older children, because they are all hard to place and the LA's frequently need to look elsewhere than their own adopters to match the child. However recently, as there are many more young children coming through the system, I have seen adoptive parents with VA's be matched with single babies/toddlers who do not have moderate/severe special needs. There are a few VA's who specialise in hard to place children only, but many VA's will now accept adopters who want younger healthier children. Still, if you are looking for a young, relatively healthy child, I recommend you start with LA's, although talk to the VA's as well, you need to go with the agency that seem the best, and if that is a VA then so be it.
VA's are generally more well regarded when it comes to post adoption support, although as always there are exceptions to this and some LA's are very good with PAS (some are dire though, talk to adoptive parents about it). Basically because VA's tend to have more money available.
I recommend you talk to every single LA and VA near you and you can attend several information evenings...they need to be enthusiastic about you, seem efficient, anser all your questions etc etc.
Best of luck to you!