Max-Fix 2 weigh around 14 kg, count on shipping/aircraft weight of 15-16 kg (33-35 lbs). Customs and import charges are small change if you bring it in "legally". In my experience customs in US never even look at the car seats and they sail straight through customs. I don't know if that also apply from UK but it's very likely. Also depends a bit on how the retailer is shipping the seat. Fedex, UPS, Royal Mail, other courier?
Checking the seat as luggage is fine. These are car seats which have passed the ultra strict Swedish Plus Test, where most foreign seats end up in pieces on the floor, with a brutal crash puls and 40G forces. It's not delicate glassware.
This does in general work very well when shipped in a car seat bag with bubble wrap and extra padding of clothing. When my kids were young we always brought two Britax Hi-Way with us on trips all over the world. Lots of those to US. Did 60+ flight in a few years and it worked great.
But HW is a very light weight seat with a simpler construction. Bringing Isofix seats on planes, weighing more than twice as much, is not as easy and also the higher weight can create some other issues.
Prices in UK for Max-Fix are lower than here. This has to do with the "recommended" price and also the cost of purchasing the seat from the manufacturer. Both of these are far lower in UK. It will likely be quite a bit cheaper for you to by the seat in UK and bring it with you although the VAT can probably not be deducted in the store. Check with the airline how much charges are and also if you can gate check the seat. We charge 95 Euros for shipping (around 130 USD).
Regardless of where you purchase the seat make sure the leg spacer is included. Britax have done a nice job with this one. Not only does it look good but it also creates a few extra inches of leg space for the child.
Good luck!