I ought to know where to look this up, but...
does anyone know if the Upper Tribunal can make the same range of decisions as the Social Security Commissioners could, including setting aside a first tier tribunal decision and substituting their own?

I haven't got time to cite the provision at this time, but it goes like this.

If a First-tier Tribunal ("FtT") hasn't made an error in law, the UT cannot interfere with the decision and cannot consider evidence that was not before the FtT.

If there has been an error IN LAW by the FtT, the UT can (in simple terms):

1) remit it back to FtT for a complete rehearing; or

2) remit it back to a LA (occasionally happens if the UT decides the LA hasn't made a decision); or

3) make its own decision, including the discretion to take into account evidence / argument that was not before the FtT.

All of the above is well established and, more importantly, covered by legislation.

In other words, "Yes".