All modern caravans and motorhomes should be supplied with a 25-metre flexible cable (usually orange, so that it can be seen in the grass to avoid people tripping over it) with pre-fitted 3-pin blue connectors suitable for UK sites. They should be compliant with BS EN 60309-2.

Can I plug a 15 amp device into a 20-amp outlet?

Most household electrical receptacles have a 15-amp rating, but they connect to a 20-amp circuit. You can use a device that has a 15-amp plug with either a 15- or 20-amp receptacle. A 20-amp receptacle is for heavy-duty appliances and power tools, such as air compressors.