At the ends, when everything comes up out of the ground, you can use press fittings to connect onto copper or, even better, insulated MLC pipe. If you go with copper, just make sure you insulate it properly with something like Primary Pro.

There are a few different manufacturers out there, but City Plumbing stock the ducts… you just tell them the length you need, and they’ll cut it to size and can do next-day delivery.

Also, make sure you get the proper end seals where the duct exits the ground. You basically heat it up with a heat gun and it seals the duct tight around any pipes, just like electrical heat shrink. It’s dead easy.

As for depth, 600mm is standard guidance to protect from frost and spade damage, but if you're struggling with that, going a bit shallower isn’t usually a big issue. Frost isn’t a real concern here since your flow and return will be in an insulated duct, unlike a mains water pipe. attached are some links and install guidance PDF.

Pre-Insulated Underground Pipes

INS140-3225-2HSC - Pre Insulated Pipe 140mm x 25mm or 32mm Heat Shrink Cap | Polypipe

I think we're basically going to say "follow the manufacturer's instructions",
but it would still be good to check out what those are.

I was thinking of an SWA *in* the ducting.

What you run the cable through will affect its current rating (and hence voltage drop).

SWA is intended for direct burial in the ground... the wetter the better.
You want any heat-loss from the cable to be conducted away into the surrounding soil.

Placing a cable inside an air-filled duct has the opposite effect.
The air acts as an insulating jacket. You've just made yourself an underground oven in a long tube. 😥 

When you select the required cable size, you would usually enter the figures into an online calculator.
Here's one on the website for TLC-Direct.

That will tell you the cross-sectional area of the copper conductors you will require.

Secondly, if you have any services at shallow depth, where they might get hit by a garden fork for example,
place some basic paving slabs above them.

Although buried electrical cables must also have warning tape above them,
there's nothing like having the fork hit a 40mm piece of concrete slab to warn you there's something down there!

Mostly its for frost protection but when the pipes are only just below the surface it is advisable to well insulate above 100mm thickness to slow losses and mechanically protect if possible.

Presently, I'm just exploring my options to get the water from the heat pump into the garage. 

I'm constrained by the existing services, so I'm not sure how deep I'll actually manage. 

In your case, I’d agree that shallow burial with good-quality pre-insulated pipework is a practical way forward. You could always add some additional foam lagging and a protective duct over the section that runs shallower, just to give it a bit of extra mechanical protection.

On the electrical side, the key thing is to use appropriately rated armoured cable, preferably in its own duct separate from the primary pipe, especially if it’s running alongside heat-carrying pipework. It’s also worth checking the manufacturer’s guidance… some kits include pre-bundled solutions, while others require cables to be run independently.

Out of interest, which heat pump are you going with? And who’s installing it? Always helpful context for others following your project too.

https://electrical.theiet.org/wiring-matters/years/2024/101-july-2024/buried-conduits-and-ducts/#:~:text=For%20public%20and%20private%20electrical,use%20as%20a%20protective%20conductor.