I had hoped that adding the wrap to the input table would do the trick, but it didn't.
Tabulate seems to prefer single-line headers. Hard to convince!

On the other hand: has an effect on the vertical axis - somehow:

dt = New Table( "test",
Add Rows( 5 ),
Compress File When Saved( 1 ),
New Column( "Fixtures& Cables",
),
New Column( "Fixture QTY",
),
New Column( "EST. SHIP",
),
New column ("value"),
Group Columns( "myGroup", :"Fixtures& Cables"n, 3 )
);
//-------try this: -------------
dt = current data table();
cols= dt << get column references;
for each({col}, cols,
colname = col << get name;
col << set name (substitute(colname, " ", "\!n" )));
// .... no change ......
Tabulate(
Add Table(
Column Table(
Analysis Columns( :"Fixtures&Cables"n, :"FixtureQTY"n, :"EST.SHIP"n )
)
)
);