Johnson-Mathey and Engelhart art the name-brand bars. They probably charge more because they are more likely to be easily re-sold and less likely to be questioned. I think they have serial numbers.
I have never heard of Silvertown, but I bet it is more like Northwest Territorial Mint or APMEX house bars. They are probably good but maybe less easily traded to people who might be paranoid about whether they are real or not.
I also think that one or both of Johson-Mathey and Englehart may no longer be producing bars. That may add some scarcity or collector premium.
If you want to just get silver at the lowest mark-up you should buy the house brands as long as you are dealing with a reputable dealer.
If you are concerned with being able to easily sell the bars later, possibly when things are moving fast or markets are confused the name-brand bars are probably a better choice.
If you want to have some silver to trade for supplies if things get really bad then probably 90% silver regular coins (pre-1965) or silver eagles might be better because the average guy may recognize them where he might be suspicious of a bar or round.
I have noticed that some PM dealers have different prices for what seems to be the same thing. What I am seeing is one price for a one ounce silver bar from a producer named Silvertown for $ 30.87 and another from Johnson- Mathey for $31.07 and another from Engelhart for $ 32.37. There must be a good reason for the difference but I am not sure what that is. Could someone explain the advantage of buying one over the other? Thanks in advance.