"250 kts at the SLP or 3 mins before the holding fix at FL140 and below"
You must be at 250 knots at the SLP - as the controller will assume this speed in setting distance (time) spacing from that point on.
You must be at 250 knots 3 minutes before the holding fix, if you are directed to holding, also ensuring that you meet the holding speed requirement of 230 knots (below FL140) as you enter the hold (pass the fix).
... So if you pass the speed limit point on the star above FL140 you are still required to meet the 250 knot requirement.
On Luton's arrivals there is an SLP with no speed or time specified???
Read the text notes. You will also find it is an ATC mandatory requirement to maintain a minimum of 160 knots to 4 DME.