You're not thinking of the entire scope of the issue. For example, the UK cannot legislate Huawei or any other Chinese company. You might say that that's true about the US too, and to some degree you'd be correct, but this also isn't taking into account that the US is (was?) a strong ally and this provides much more leverage over the situation. It ALSO means that IF these networks are being used to spy on citizens that there's a lower worry (still a worry, but lower). It would also mean that if this data is not being shared with the UK then this would be a violation of the 5 eyes agreement, which means the UK has more leverage over that situation.

So yeah, even if they are equal, there are A LOT of reasons to spend the extra money.