I've heard that before. People in the uk likes 120V power tools.
I have good and bad news:
Your cutting it too close on wattage. This
https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/6641871/ might be a better choice. A 400W resistor was cheaper than a 300 W resistor. You don't have to solder if you use the wire leads.
250w is a lot of power to be wasteing, but it's actually good news too.
The RF switch is sold as a "Definate purpose contactor", so if you were switching 0.1 Amps, the relay contacts MIGHT become unreliable
A relay has what;s called a "Wetting current". That's how much current you have to minimally have for good reliability.
250W has a really good chance of not exceeding that.
I don;t LIKE the design, but it should work.
A note:
2 resistors of the same value in parrallel is: Req=R/2 so, two 8 ohms is equal to 4.
Wattages add: so Two 100W 8 ohms in parallel is 200W at 4 ohms,
Resistors might be found at surplus places,
250W is too close to 242. I'd be happier with a 300W. resistor.
i was thinking you were using smething like this
https://www.amazon.com/eMylo-Wirele...nsmitter/dp/B01A6VO79M/ref=asc_df_B01A6VO79M/
But note,, no real info on the relay contact ratings.