Richard,
YES!
Until very recently, you could book RFS on IB with Avios.com and ba.com - but now you cannot.
You cannot book RFS with IB+anymore, but I think IB taxes etc are lower for IB flights than BA taxes etc.
Obviously, you are still better with BA's RFS. But sometimes, needs must.
IB+ are also much better for long-haul (to IB destinations) with very little in taxes and fuel surcharges - albeit you have to get to MAD.
But IB+ are very poor in comparison for non-IB flights (require a return ticket and not so hot on cancellations, I think).
So, in summary, open an IB+ account.
Credit some miles - there are a couple of ways of doing this quickly.
Wait 90 days (which is why you should do it now, because you never know whe you might need IB+).
And you are set to go.
Transferring avios between BAEC, avios.com and IB+ takes just minutes. So it doesn't matter which scheme you keep them in.
But it is well worth having the flexibility.