I have always coveted a H&H Royal in .375 and a Westley DL in .450 to .500. To me these are quintessential English DRs suitable for hunting and with collector/investment appeal as a bonus. I may be wrong, but for their value to increase depends on 2 factors, the cost of new rifles and the condition of the older ones. For a new rifle to increase in value would require the rifle to be in virtually unused condition and for the price and waiting time for new rifles to put off impatient investors.

For older rifles, such as rising bite Rigby's, the investment value lies in their scarcity. While the name has been irrevocably tarnished by the Mexican efforts, a London made rising bite is a true investment piece due to their scarcity. No more are being made and they were costly to make to begin with. The Rigby name has also been immortalised in print and they were the originators of the NE double rifle. However, finding a rising bite Rigby in investment grade condition is very tough. There are many pieces that have been rechambered or rebarrelled, particularly the smaller bores, but I have seen very few exceptional big bores. That is why I am surprised that the 1902 mint .450NE that was recently sold at Julias didn't raise more interest. Credit crunch I guess, but the ideal time for alternative investments...
Purdeys are exceptionally made and the design has the nice feature of cocking on closing (whereas H&H and most others cock on opening) which means that when taken apart the springs are at rest without needing to be fired against striker blocks. However, I have seen very few large bores and would have to bow to other's expereince. The rarity, build quality and name should be sufficient to ensure investment returns, but you'll have to pay for it.

There are others of course, find a nice Jeffery under lever .600NE and it will put a smile on your face! A mint Alex Henry black powder DR would be fantastic too but good luck finding one! At the end of the day it boils down to condition and rarity. Otherwise, buy a DR suitable for hunting in the best condition you can, either new like a WR or if second hand then the earlier the better. Or some bars of Gold!