I've found both Easyjet and Jet2 to be realistic and honest with their baggage policies.
Not so much of the 'slot it in the frame', more of a quick glance to check that it complies with their terms.
For a summer trip to Auld Scotia you'll not need much anyway, enough clean underwear for the duration, a warm fleece and a raincoat plus a midgie hood if venturing to the north west.
Summer clothing is always available on the sale racks as it's ordered more in hope than a basis in reality and vendors are keen to move it on. :-)