Did not see him in1958 at the Odeon but saw the Crickets with Bobby Vee at the Demonotfort Hall in 1960 absolutely brilliant and very very loud too. No 100 watt Marshalls back then either.
Sun never released an LP, only singles. Elvis did not record for Capitol, his contract was purchased by RCA. The first LP in this country was Elvis, now referred to as Rock N Roll no 1. His UK singles initially were on HMV ( I have Lawdy Miss Clawdy/ Trying to to get to you).
I would also think it is a 78 record.
This era I remember very well. Saw the film Blackboard Jungle in Clacton while on holiday. Bill Haley and the Comets played the signature tune and the film as I remember starred Glenn Ford, Sidney Poitier, Vic Morrow and Sal Mineo. I can't remember anybody else but the signature tune really started proper Rock n' Roll in this country and made Haley the main man for a while. The film you refer to came a little later and Haley and the Comets were the main stars.