Just my 2p worth.
On the face of it, it looks like it
may be suitable, but when you take a good look............................
About this item
- Product 1: WEATHERPROOF AND ROBUST: From Masterplug, the double inline socket extension lead, has an IP54 weather and dust proof rating, giving you a safe temporary outdoor power supply
It is only designed to be used for example, to be run out, spike pushed into the soil while you cut the grass, and if it rains, you can leave it outside as the plug is connected under a weather proof cover (Orange thing, not green box) just hope lawnmower doesn't rust.
There is no mention of RCD protection.
The orange flex, is just that, orange flex, it is not capable of withstanding cold temperature (You would need this if it is to stay outside all year, which it is NOT designed to do)
The orange flex has no mechanical protection, meaning it can easily be severed, nicked or similar, another reason it is only for temporary use.
If you do the job properly you will save money in the long run.
Are you putting in a cheap polythene liner in the hole? why not, it will hold water? Obviously you will be using a decent quality of liner, so why not do the electrical part right first time too?