Wide back box with central 1-gang mounting
You can get various conversion patresses which will fit over a flush 1-gang box and present a surface 2-gang box, eg 



and conversion sockets, eg



If you want to fit a single socket on a double box you'd probably need to use a Euro module faceplate, Euro module socket and blanking plates



If you just need a decorative cover then 'finger plates' are available in white or clear

https://cpc.farnell.com/pro-elec/pel00515/finger-plate-for-light-switch/dp/PL10027