The class is set before the steel is ordered
Execution class is part of the design specification. The engineer, or the execution specification they write, sets EXC1, EXC2, EXC3 or EXC4. The fabricator does not pick a class to make the job easier, and a buyer should not assume every workshop is certified for every class.
EXC2 is the class that covers the majority of building steelwork in the UK and Ireland. Higher classes bring tighter controls on welding, inspection and traceability, and they have to be specified. If the drawings are silent, ask. Do not leave it until the frame is in the workshop.
What CE marking is actually saying
CE marking to BS EN 1090 means the fabricator has a factory production control system and a certificate for stated execution classes. It is not a comment on whether a particular frame was a good design. It is evidence that the workshop is allowed to make structural steelwork to that class.
Steel Structures (NI) Ltd fabricates structural steelwork CE marked to BS EN 1090 up to Execution Class 2. Certificates are available when a main contractor is building a supply-chain pack. The detail sits on the CE marked steelwork page.
What to ask for at tender
The current certificate, the execution classes it covers, and confirmation that this package sits inside that scope. If the specification calls for something above EXC2, say so at enquiry stage. Alongside that, procurement teams usually ask for CHAS, SSIP, Constructionline and Builder's Profile.
If you are still shaping the frame, the useful conversation is earlier than the certificate. Design and pre-construction is where execution class, connections and programme get settled before the price is frozen.