In Germany that will result in them getting some MPs in a separate party, and their centre-right party will do a deal with someone more central (their centre-left party last time, probably the Greens and the liberals this time around). Total influence of the far-right: almost zero.
In Britain this resulted in the Conservatives basically implementing the manifesto commitments of UKIP in order to stop their party fragmenting. Total influence of the far-right: Brexit.
This is _entirely_ down to the voting system. Germany has basically had the equivalent of a Lab+Con coalition for most of the last ten years, and the parties are constantly moderated by having to work with each other. Even France's two-tier presidential elections allowed people to vote for a relative outsider.
In Britain you end up with two main parties which spend all of their time scrambling to keep both wings together. It's a political system which, frankly, encourages extremism by making the more central politicians reliant on the further-out wings of their parties.
Original post on Dreamwidth - there are comments there.