Can I use the mains lead from my old macbook 2015 on new power adaptor so I have extra reach.

I'm not sure I understand what you are asking. You can certainly use the grounded power cord if you wish. Apple also sells them separately:

Power Adapter Extension Cable - Apple

That is the US / North American version but a UK version is probably available if you load that site in the UK or anyplace else beyond North America.

You can also use any common electrical extension cord available at any hardware store. They cost about 1/10 as much as Apple's nice white cord.

Apple why have you stopped supplying this anyway - a £2.5k plus lap top and you scrimped on the power cord. The charging block is really awkward to plug into UK sockets ...

Apple is not here, so they will not see your question. To contact Apple for the purposes of asking that question or any other, Contact Apple.

If the power adapter on the 2021 MacBook Pro disassembles as in this photo, then yes, you can use the mains extension from the earlier adapter on this one without problem.

Yes, you can.

All the AC power extensions cords we have seen to date are interchangeable.

They don't include a power cord with them in the US either.

• Genuine Apple Power Adapter Extension Cable £19
• Generic UK extension cord Tesco 4Way Extension Lead £5.50