You can plug what you like into it provided that you don't draw more than 13 amps at any one time. In other words, if you have a 3KW kettle and a toaster plugged in, you only use one at a time. If you have a 1KW toaster, a 1KW coffee maker and a mixer plugged in, you can use them all at once.
maybe you have a faulty batch of adapters? Either that or your grill and toaster are drawing much more power than can is acceptable and you have overload.
Trybuying a new one from a reputable supplier and make sure you only use one appliance at a time. Either that or you switch the kitchen about and find a socket each for them. Share the toaster with the radio or something else that has no heating element.
Next, 3kw will be the maximum load of everything you plug into it at one time, summed together. Probably wise not to go near the maximum.
You may wish to investigate what it was that caused the browning/melt. Either it is arcing inside or something is taking an abnormally high current and may have a fault.
Two things to consider here, Joko. One is that good quality double sockets are largely designed to handle a continuous load of 20 amps (total - both sockets loaded.)
But they must be of good quality. That's the second thing. There are loads of cheap imports that will not take this load without damage. Even MK advise a 20A total load.
This is in contrast to the (theoretical) notion that 2 x 13A sockets mean a capability of 26A!!!
Even without the possibility of loose connections/arcing etc, this is worth remembering.
Grills/kettles/toasters etc have relatively high-load heating elements. They should each have their own outlet.
Do consider having extra sockets fitted alongside the existing ones. A much better option.
Sparks will be on to tell you I am sure. But those extension leads really are only for small loads and not good as a permanent solution. You are operating electrical goods on what is a temporary spur, which does not offer the load sharing protection that a socket in the ring main does. Do as builder says and get permanent extra sockets fitted that are part of the ring system and are of a decent quality fitting. That fishy smell you get when a socket or plug or light fitting is overheating is not an accidental result.