bro, if that car is 5 more than 3 year old and from UK than just pass me the uk original registration number and document ref.number(uk log book first page) and i can check the mileage for you for each year.
but if that car is less than 5 year old than you might need to check it via special scan machine which is available here in UK but quite expensive.
there is one more cheaper option....
check the car steering wheel for wear tear, driver seat,gear knob,door handle,check front bumper/grill/bonnet for motorway chip,
and if owner allow you to open the engine plactic cover than do that and you will clearly see signs for high miles or low miles,as with only 20k on clock you should expect shiny engine with no burn metal marks at all which you get with high miles.
hope this will solve your problem a bit.