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The stop cock needs to be as close to where the pipe enters the house so wouldn't look to have moved the stop cock.
The earthing should again remain as close to where the pipe enters the property/where the copper starts, so no need to extend.
The length of copper is not the issue from drinking etc, your builder will probably use plastic anyway. The issue you will need to consider is about flow rate.
By tapping off the bathroom, you may a notice a drop in pressure when using the bath/shower, and the kitchen tap.
Secondly they will likely use 15mm pipe, where you should use 22mm to allow for sink taps, dishwashers etc to be supplied it's a good flow rate.
So I would run a two new pipes in 22mm from boiler and mains to kitchen, then spur off the 22mm in 15mm to each appliance.