Ouch.
Did he gI've you any timeframe?;I've read that they're one of the better places, but always busy and turnaround can take a while sometimes.
I've no doubt, based on the comments I've seen in other forums that Ted Porter's shop does excellent work. But there is nothing "BMW-ish" about changing seats and setting up new valves, any shop with decent work practices and reputation should have no trouble at all.
Having said that I do remember someone in this group posting a "reconditioned" cylinder head that should have been inserted up the fundamental orifice of the butcher pretending to be a machinist that did it.
If you can't wait for Ted Porter (or afford him) ask your local engine rebuilding shops if they have experience in setting new seats into alloy heads that are suffering valve recession, and how they propose to do the job. Select the one that says they will first replace (or recondition) the valve guide so that they have a known good center, that they will then remove the old seat by the least destructive method that does the job, then send the head out to be welded up (the seat bore will have been damaged by the recession and probably further damaged in the removal process), because they have a known good center in the new guide they will then machine a new bore for the new seats you will supply (buy BMW they are cheap amazingly) and then put the head in an oven and the seat in liquid nitrogen (or dry ice or liquid oxygen, whatever they usually use) prior to pressing the seat into the head. They should insist on you supplying new valves (take a deep breath and buy BMW part, or if you cannot justify that, Intervalve from Motobins are good. Followed by a 1st class three angle valve job and you are good to go.
A good shop will also honestly tell you that there is always a risk of failure with pressed in seats, sadly the cure is to simply do them again and hope it works 2nd time round (with quality tooling and people who know how to do it, failure rate is low). If the shop suggests using "screw in" seats, walk away, don't bother arguing or asking further, just walk away, these are a "quick and dirty" fix used by sweat shop type rebuilders who accept a high failure rate as part of the cost of doing business.