Micrograin carbide high helix tools for aluminum work well, but they have to be new. A tool that has touched metal will not perform as well.
Use the same rpm that would be used for aluminum and take healthy cuts, maybe two or three times the chipload that would be used for aluminum, and as mentioned make sure your part is clamped well.
As far a clamping goes just relying on friction to hold UHMWPE is dicey, it is very slippy and not very rigid so it will pull out of vise jaws. Some kind of keying is necessary to hold it under aggressive cutting. For blocks if possible prep the part with a dovetail step and use custom jaws which form a mating dovetail so the material is gripped and held down.
When working with sheet material held down with bolts it is sometimes handy to have a sheet of 1/8" or 1/4" aluminum over the top and machine through it. This spreads the bolt pressure over a larger area and stops corners lifting under the action of the cutter.
An open mind is a virtue...so long as all the common sense has not leaked out.