I don't think travel from Wymondham to London is permitted via Peterborough on a walkup through fare. The only available walkup through fares are routed either via Cambridge or via Norwich. Whilst Peterborough passes as an origin Routeing Point even when entering the via Norwich route, the via Norwich route restriction still applies and so, in practice, it is not possible to use any permitted routes from Peterborough.
There are Advances available from Wymondham to London, but they are all routed Greater Anglia only so travel via Peterborough is de facto barred.
I'm not sure what you mean about which fare you should use - simply that, if you wish to travel via Peterborough, some online booking engines will tell you it's not possible at all, others will sell you a combination of tickets (e.g. NRE and other SilverRail split-ticketing enabled sites such as Chiltern, as well as TrainSplit), and if you ask at a ticket office they are obliged to sell you a valid combination (though it need not be the cheapest combination possible).