Hi Duncan
I have a little knowledge about re-registering an EU vehicle between member state countries as I'm in the process of doing it for a UK registered Citroen car into France at the moment. One of the first and most important documents your'll have to obtain is a EU Certificate of Conformity. It basically affirms that the car was manufactured to EU standard specifications and complies with such things as EU emission regulations. You get it usually from the Manufacture's Company Head Office in the country the car is currently registered in ( ie in your case Renault Germany ) . Some companies charge for the document and some don't - it's a bit of a lottery and I'm not sure if Renault do or not, if they do expect to pay around 150 - 200 euros, it's not cheap. There are internet sites that for a little extra will do the job for you and in any case you'll need to send off copies of the current registration document, proof of ownership, proof of identity (passport) and proof of current address ( utility bill within the last 3 months). After that it's a matter of following the process through eg MOT, UK insurance etc. The DVLA has a good web page on the way to proceed. On a 1999 vehicle it is unlikely you'll be subject to VAT but I think the DVLA site covers that aspect also. Best of luck -I'm told the UK system for re-registering is quite simple and straight forward unlike here in France where it's a bureaucratic nightmare.
As for the RHD headlamps I suggest E-bayUK or there's any number of online vehicle dismantles in the UK that might be able to help.The twingo headlamps are pretty basic and shouldn't cost a fortune, probably no more than £60 for a pair secondhand. The twingo was manufactured as a right hand drive version for the UK but there were relatively few build compared to their popularity on the continent, so you may well have to hunt around a bit but I'm sure there's a pair out there somewhere just sitting on the shelves in a dismantles warehouse
Claude