I think the question is: with blinds at 100/200, after UTG goes all-in for 250, what is the minimum amount that any other player must pay to stay in the pot -- i.e., can they just call the 250 or are they required to "call" the amount of a full raise of 400?
The rationale for requiring 400 would be similar to the situation where blinds are 200/400 and the BB is all-in for 250 (less than the amount of the big blind). In that situation, if any player wanted to stay in the pot, they would be required to call the full 400, even though the player in the BB had only posted 250. One could argue that since we require players to call for the full amount of the blind even when a player posts less then the full blind when all-in, we should require players to call for the full amount of a minimum raise even when a player wagers less than a minimum raise amount when all-in.
I think everyone who has responded on this thread -- myself included -- would not follow the above reasoning but believe that players may call the 250 wager and are not required to "call" 400.
I also wanted to note that no one who responded addressed the house rules that the original poster specified: "All raises must be at least double the previous bet, with the exception of an all-in. Pre-flop, the initial bet is the big blind. Therefore, the minimum raise would be double the big blind, with the exception of an all-in."
The OP seems to be taking the position that the rules specify that the minimum raise pre-flop would be double the big blind = 400, and since UTG wagered an amount more than the big blind it was a raise, which by definition was required to be 400, so everyone else must thereafter call 400. Not only is this interpretation not the generally accepted one, but the house rules themselves specifically include an exception to the "minimum raise must be double" rule in the case of an all-in -- i.e., the rules allow a pre-flop "raise" of less than double the BB when a player is all-in, so the 250 wager would be an acceptable "raise" under the house rules.
The house rules quoted are also somewhat confusing to me. When they say that "all raises must be at least double the previous bet" is that really what they mean? I think they mean that "all raises must be at least equal to the previous bet" (which is different than the normal rules that require all raises to be at least equal to the previous raise). Otherwise, read literally, with blinds at 100/200, the min raise would be to 600 -- a call of 200 plus a raise of at least 2x200.