The main difference is that distance to go is fixed block, wheras moving block is moving block (shocking stuff
hock: ). Both are forms of ATP or ATO. The latter is more suited to high capacity metro lines where all trains are fitted with the system and have similar, or better, identical performance, and is very suited to ATO. The former is just an anhanced for of ATP, which rarther than using a blocks to go system like BR-ATP or TVM, it tells the driver or train exactly how far until it needs to stop. One of the big differences is the accuracy of information on train speeds and positions. DTG pretty much works like a block based system but using small blocks, and can still use track circuits. Moving block on the other hand needs very presise data about the exact position and speed of the train infront. One big issue, that often makes moving block unviable, is that it's difficult to integrate into existing signalling systems, which is essential if trains are going to run that arn't fitted with the system. This would have been a problem on the WCML, since to make it viable all trains, not just the Pendolinos, would have had to be fitted. If not, you end up reverting to a normal, fixed block system around none fitted trains, since these neither have the ability to tell the system where they are, or to tell how far ahead the train infront is. It only really fitted with Bransons original idea of a totally segregated 140mph railway for his Pendolinos.
ETRMS actually offers all 3 ATP systems. Level one is a basic blocks to go system, which can work with or without lineside signalling (they could be replaced with TVM style section markers, although again it means all trains must be fitted), level 2 is distance to go, and level 3 is moving block. Level one has been critisised for reducing capacity over conventional signalling, and level 3 is expensive, un-practical on a mixed system and mostly unnecasery, so level 2 is the most likely option to be fitted, and AFAIK this is what is going to be tested on the Cambrian Coast.
140mph running though wasn't dependant on moving block, just cab signalling, so hopefully once ETRMS is fitted, the WCML will finally have 140mph running, along perhaps with the ECML (by the time it happens, the 91s and Mk4s will be up for replacement, and I'm sure the replacements will be 140mph capable at least.