Vietnamese Restaurant in Chinatown is really not so good. I recommend trying one of the many Vietnamese restaurants clustered on Kingsland Road E2, and Mare Street E2. Kingsland Road is a 5 minute bus ride, or a 20 minute walk, from Liverpool Street station. My personal favourite is Song Que on Kingsland Road.
I don't know about the cubes, but as it happens, I made my own Pho at home over the Easter weekend, and I am thrilled with how it turned out. I got 6 lbs of beef bones from my local butcher (although I am lucky to have one) and par-boiled them for a few minutes, then simmered for 3 hours in 4 quarts of water with star anise, cinnamon stick, salt, fish sauce, cloves, and onions and ginger which I roasted in the oven for a while first. I also simmered 1 lb of beef brisket in the pot, but removed it after 90 mins. I cooled it overnight and skimmed the fat, and have frozen it in 2 cup blocks. Not the quickest recipe, but I now have instant pho whenever I want it, to which I add spring onions, rice stick noodles, cilantro/coriander, slices of the cooked brisket, and even some bits of the gelatinous tendon which I stripped from the beef bones before discarding them.
Call me Vietnamese!