Hi welcome to the forum. The postcode has no effect on the strength of the signal only the channels that are allocated to the local channel numbers. Whatever you use you have all the BBC regions and ITV1 London is in the epg at 977 using all postcodes.
It makes a difference on Sky boxes because the viewing card gives you access to some ITV regions that you cannot get on a Freesat box (most likely not for long)
If you want to give your box a degree of exclusivity choose one of the following 
http://uk.answers.yahoo.com/question/index?qid=20060730053644AAIpuEf
There might be a local reason why a specific postcode gives a more reliable signal for ITV1 on 103, only local knowledge can answer that one.
The customised firmware allows you to add multiple ITV1 SD and HD regions to your epg (London or Granada for HD).