Just bought an old Golf with S5UGR private plate and trying to understand how desirable this might be to some S5 owners if I decide to sell it? Might read it as "S5 SUGAR" I suppose so some might find it attractive? Any advice greatly appreciated.

Welcome Rafael, and thanks for posting your question.

If we translate desirability into value, the route to that would be via getting a valuation from specialists such as Registration Transfers - there are many more. They will value it on a ‘Return to you’ basis and do all the advertising for you. 
My experience is that these can take an age to sell - if ever- but it’s well worth a try. One question worth asking yourself is :- If it were so desirable, why didn’t the previous owner retain it to reap more funds from the sale? 
As I understand it, the donor Golf would have to have a current MOT, and I believe ( but please check) will have to have been continuously taxed for the past 5? years. 
Please let us know what you find out Rafael.

Kind regards,

Gareth. 

Thank you very much for your comment, Gareth. I've never had any experience with private plates before so your insight is very much appreciated. I did request a valuation from a couple of specialist services and both valued it at £700-800 (what I would get in case of sale after their commission is taken). The car has been in constant use (judging by its MOT history) so had been continuously taxed I believe and has a current MOT. As to why the previous owner didn't retain the plate for themselves - it's anyone's guess I suppose. Also, thank you for sharing your experience that it may take a long time to sell - this is something I obviously need to take into account.

Kind regards,

Rafael

Many thanks Rafael. So it would seem that it would be possible to place this number on a Retention Certificate  ( cost around £110?), then select an agency to list it for you. Yes you gamble that money, but it does retain the number. 
Kind regards,

Gareth.