Is it safe to use a 50 metre (164 foot) mains extension cable?
Hazards

I believe the main issue is that they can get warm if you don't fully extend them and then draw a high current through the coiled wire.

See Leads &amp; Extension Lead Safety



RCD (Residual-current device) or GFCI (Ground Fault Circuit Interrupter)

Most advice is to use an RCD with extension cables over 15m. I use a plug-in RCD. You can buy replacement outlets(wall-sockets) that incorporate an RCD.

I suspect this advice is simply because the length of cable increases the likelihood of accidental damage to the cable (spades, shut in doors, etc)



Voltage drop

The voltage in the UK is nominally 230 V with a tolerance of +10% to -6% (in much of the EU it is +/- 10%) - In practice most wall outlets are likely to be at 240V as this is the standard voltage prior to EU harmonisation and is within the tolerance.

This means that equipment must be able to work with voltage as low as 216.2 V.

If plugged into a 240 V wall outlet, your 50m cable would have to lose nearly 24 volts before it dropped to a level below the UK tolerance in supply.

That means, at 13A and 50m the voltage at the end of the cable would be OK so long as the cable had a resistance of less than 30 mV per amp per meter. This means you want a copper cross-sectional area of at least 1.5 mm². Ref, Table

The details for your specifc product don't mention the conductor size so you can't do the calculations for yourself.

You can check the voltage drop easily with a "kill-a-watt" type device or a suitable multimeter or voltage-tester rated at least Cat-II 600. If the drop is too high you have the statutory right (in the UK at least - SOGA 1979) to return the cable as unfit for purpose.

Generally, when voltage is low, appliances stop working or work poorly (dimly, slowly or intermittently) in most cases this wont cause a hazard.