On the pipework I've got 17mm measuring on the outside diameter of the main pipe (the part that's not the old tap). So at that end the connector will be the 15mm you mentioned? Pipework only comes in 2 diameters?
Basin and sink taps are 1/2" BSP ( British Standard Pipe), Bath are 3/4" BSP.
Also, BSPP means parallel thread, BSPT means taper thread.
If you are replacinga pair of basin taps you should be able to without any work on the pipes.
Also, if this is what you are doing it might be an idea to replace the fibre washers in the tap connectors. (As ilustrated by the crudely drawn arrow.)
These are Ebay items used as examples. I do not endorse them, but use them to show what you can get for not too much money.
The last ones claim to offer a 5 year guarantee.
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