Travelling from London to Amiens France. Which website is best for booking trains?

I would use: https://loco2.com/
They do through bookings across railway companies. They are a UK company and give prices in £. They are currently quoting £80 to £90 for a random day in May fro London to Amiens.
Eurostar may be able to do this booking, but my experience is that they have limmited add-on destinations.
Doing it as one booking means you know you have allowed enough time between trains, and if you have a through ticket and you miss your connection because the previous train was late, they will allow you on the next train.

Thank you I will check them out. I was also looking at raileurope.com. Anyone heard of them?

Why not just book the Eurostar to either Lille or Paris and then train from there to Amiens through SNCF. Eurostar arrives at Paris Nord and train to Amiens leaves from Nord also so there would be no station change. I'm not sure about Lille but I think there might be a station change there between the Eurostar and the regular train to Amiens, so Paris might be the best bet.

Thanks, that might work.

raileurope.com is a ticket reseller in the USA which is regularly lampooned on this forum for markups and limited choice. The same train I saw on https://loco2.com/ for £83 they are charging US$ 156 (= £ 103).

It is not necessary to go all the way to Paris and double back. Some of the options I saw on loco2 involved a change at Lille or Calais. But it is not necessarily much quicker, as the train from would not be a TGV.

In most cases it is usually cheaper to book with the railway company running the train: http://eurostar.com/ and http://www.voyages-sncf.com/ The further away the ticket reseller is, the more likely they are to want to add their margin.

Thanks, that looks like a good website. I looked up several reviews and seems better.