Plastic taps from Screwfix - £4.28 per pair inc vat.

Well it made me laugh!

Nothing in the screwfix catalogue that looks like it might work as an outside tap though. An insulated break in the pipe is more sensible, the perception threshold is a few hundred microamps to 1mA for most folk, so some tens of K ohms in series would be enough -  a 6 inch length of PEX would afford that in many parts of the country.

Nowhere in the UK has a higher tap water conductivity than 2500μS/cm at 20°C , this being the test limit that nowhere failed for the report in the Chief Inspector of Drinking Water - see page 17.  This is actually quite a low limit (it would mean a 1cm cube of water is 400 ohms across opposite faces) but sets a worst case for estimating what sort of pipe lengths you may need. Conductivity is a proxy for total dissolved solids (TDS) when a proper chemical analysis to determine exactly what is dissolved is not worth it, really it represents a sort of average free ionic charge density. (indeed some are calibrated as TDS, either %, ppm (normally by weight) or sometimes milli-grams per litre. example) 1000ppm is normally taken as about 1500μS/cm at 20°C, or about 670 ohms across the faces a 1cm cube.