Decided to go ahead and install the outside tap myself. Here is a before and after of the internal pipework:
I didn't use a separate double-check valve as the outside tap came with one built in now I appreciate that this may not be acceptable for the latest regulations but for me I think it's fine. I didn't sleeve the 15mm pipe either as whilst I again accept it isn't in line with regulations but I gathered it should be fine especially given the thousands of homes that had outside taps fitted before sleeving was the new norm. If anything goes wrong I'll accept it as my own fault.
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