OK, Rheinmetall (as RBSL) are doing the upgrades in factories in the UK. However, the L55A1 (Rheinmetall Rh-120) smoothbore gun replacing the Challenger 2's non-NATO compliant L30A1 rifled main gun and the Horstman suspension is German technology. It's not like it is a homegrown MBT using all British technology.
Besides, the upgraded tanks are not going to be ready until at least 2027 and are not guaranteed to be fully operational until 2030! Surely, the USA could supply us with new Abrams M1A2 tanks long before that?
Furthermore, the Defence Select Committee said in a report entitled
Obsolescent and outgunned: the British Army's armoured vehicle capability, that 'Despite having spent around 50% of the allocated budget (£800 million), the programme has yet to place a manufacturing contract. The programme has a current in-service date of 2024 (originally planned for 2017) and is some £227 million over budget. After a decade of effort, this abject failure to deliver against both cost, (with an overrun now totalling over a quarter of a billion pounds of public money) and timescale (ISD seven years late) is clearly totally unacceptable.' So it doesn't look like the Challenger 3 project is good value for money anyway.
It might be a sign of our decline as a military power to rely on US MBTs, but at the end-of-the-day, if we have to fight Russia/China with them we are going to be doing it alongside the yanks anyway.