Not trying to be rude, but someone who is complaining about PCB turnaround time, but has also never soldered SMDs sounds odd. You need someone who knows what they're doing on staff. Either hire that person or become that person.

It's been a couple decades since I was involved in the H/W business, but there are usually tricks you can use to assemble even very fine pin-pitch devices in your kitchen. Heat gun, microscope, etc. I'm guessing there are 100 YouTube videos on this. Assemblers will tell you it needs ZYX special machine that only they have, but actually you can do it yourself, albeit slowly and with defects here and there.

Back when, there were decent PCB houses in the UK (up in the midlands typically, not near London). Perhaps they're all out of business now but you might go looking for one of them vs back and forth to Asia for prototypes.

Edit: a quick search suggests that there are many UK manufacturers still alive. E.g. [Forward, Newberry, Leicester, Stevenage]circuits.

LGA-12 is totally doable yourself, make sure you get solder stencils with the boards and buy yourself a cheap reflow oven. Then you just put solder paste on the board using the stencil so it's in the right places, place the components, and the oven melts everything into place.

Buy yourself some laser-cut paste-mask stencils out of Mylar (https://www.pololu.com/product/446/). Get the room-temperature stable solder paste from Chip Quick (https://www.chipquik.com/store/index.php?cPath=470&osCsid=il...). Buy a cheap temperature controlled hot plate (https://www.amazon.com/Soiiw-Microcomputer-Soldering-Preheat...). Get some cheater glasses with 3x or greater magnification, and some good tweezers. Place parts by hand, and solder them within minutes on the hot plate. Maybe watch a Youtube video or two. Easier than it seems. I've done LGA-12 with a setup like the above (a ST LIS2D12 3-axis accelerometer). Also, goes without saying that you should get a soldering iron with a fine tip for large part rework, and possibly a hot air pencil for reworking the tinier parts.

For the parts with pins you can't reach with a probe, be sure to leave a way to test connectivity of the pins with a DMM. For the super tiny parts with the hidden pads, I like to have a series resistor that I can remove, and then use a DMM to check that you can find the ESD diodes on each pin of the package.

Lga12 with soldermask stencils and a hotplate doesn't work? Should be doable for prototype quality. Might want a computer controlled pick and place with a suction arm/microscope but it's definitely feasible. Look up some of the gpu repair guys for inspiration, they often do some insane bga parts (nand chips) by hand.

Getting the tools and skills to do PCB assembly and SMD rework seems like a no-brainer (indeed I'd say essential!) if you're aiming to do serious work with building hardware. Not something I've done myself but soldering an LGA-12 doesn't look too hard with appropriate tools.