Tricky one now, what happens if you are on a heading fast approaching GWC (slp into Gatwick) thinking "great, we're on a heading and we haven't been asked to slow down, lets do 340kts", then 2 miles to go you get own navved to GWC. If given an ATC speed restriction that compromises the SLP, then fly the speed assigned, if given no restriction then fly according to the SLP or ask ATC if the SLP still applies.
isobar - if cleared passed a point on a STAR that has a restriction then the restriction will need to be re-issued by ATC if they want it to still apply. We're all aware of what the restrictions are and will give those that we really need to apply. Think of it the other way around - inbound to LGW on the WILLO 4C, a restriction is published at DOMUT FL270. You've been given FL280 for initial descent and are approaching DOMUT, you aren't going to continue without clearance. Level restrictions are there purely for ATC separation issues and are printed for planning purposes only so you know what to expect, however if they need to be applied then the controller will state it in the clearance given.