First, phone the electoral registration officer for the area and find out what wards are included in the new constituency.
Next, go to the public library and check the voting patterns for these wards for the preceding local elections. (To be on the safe side, check back two local elections lest an out-of-kilter result be present.)
Because wards are seldom split in boundary redistributions, this will allow him to assess with reasonable accuracy the electoral pattern of his new constituency.
However, writing "anyone but ABC" on the ballot paper (assuming there are more than two candidates) would result in a spoilt ballot for lack of clarity.
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