The wall and maximum / minimum thermometers are often available in good hardware stores and many garden centres, in a variety of designs. Simple aneroid wall barometers can sometimes be found in hardware stores. The cost for the wall thermometer is minimal and the maximum / minimum thermometer quite modest.
Such national stores as John Lewis have a limited range of barometers and indoor thermometers.
I would recommend that before any purchase is made some or all of the following websites should be viewed to see the wide range of instruments available and their relative costs. The sites are in no particular order.
metcheck.co.uk – wide range
russell-scientific.co.uk – wide range
brannan.co.uk – wide range
thermometresuperstore.co.uk – thermometers
weather-station-products.co.uk – wide range
amazon.com/weather-instruments – wide range
weathershop.co.uk – wide range from individual instruments to complete professional weather stations
N.B. European Union legislation prohibits mercury thermometers and barometers being sold to members of the public or to a company.