Varnish recommendation please

I have spent an enjoyable couple of days putting together a model only to wreck it right at the very end. The model was printed on 160 gsm stock using an inkjet printer. I decided it looked good enough to keep and varnished it as I have done in the past. Well, I guess in the past I just got lucky with the varnish I used (wish I could remember what it was). This time I was horrified to see the ink run and the whole model sag.

So, my question is what is the best varnish I can use to avoid the situation again? Something I can easily get in the UK would be a bonus!

Thanks one and all.