Only the Baxi gives you the possibility of retaining all the existing controls and no re wire as it has no pump over run. All the modern boilers except the Baxi require pump over run as they need the latent heat taken away. The design of the Baxi heat exchanger allows it to take the latent heat. The old boilers had bigger channels so this was not a problem but they were mush more inefficient.
Pump over run means that after the boiler burner flames go out when the max flow temp is reached the pump remains on for a 2 - 5 minute overrun continuing to taking the heat out of the boilers heat exchanger. Boilers without the need for pump overrun require a 3 wire cable (live, earth and neutral), boilers with a pump over run require a 5 wire cable (live, permanent live, earth, neutral, switched live).
Anything boiler cheaper than the Baxi's is going to incur rewire costs and require a new programmer so it may not work out cheaper in the end. I look at the Ideal they have some decent cheaper boilers and see.
We do not fit the really cheap boilers it's not worth the grief, too much of a gamble, too many problems too early poor warranty service.