What is the best mod for a brand new ps2 v5??

The NEO2.x chips are stuffed for booting DVD-Rs. There is a really convoluted process for a particular version of GS2 (2.03 I think) - but it's just not reasonable to have to use this method.

So, my wholly biased advice (well supported by other anecdote on the various forums) is the OrigaChip for US. You'd need to buy it from Lik-Sang as there's no US distributor. Or you could order it in Europe from origachip.com.

This chip will boot DVD-Rs with DVD Region X (simple swap) and will also boot import originals, in your case the DVD games you can buy in a UK shop.