I would move some or all of the money. With £30K savings, you have a 20% deposit, whereas you can get a much better mortgage rate with a 40 or 50% deposit. That's true no matter how good/bad your credit rating, and it's possible that with a bad credit rating you may not even be able to get a mortgage with a small deposit.
Also, you will almost certainly save significantly more by paying less mortgage interest compared to the interest rates on your savings in the Netherlands.
Shop around for a cheap option to transfer money. I had a quick look at Transferwise (no affiliation, they just happen to have a convenient calculator on their website), and the all-in cost for a large one-way transfer seems to be about 0.5%. I think you'll more than make that back in terms of savings on your mortgage.
If you intend to move back to the Netherlands at some point, then you are taking some exchange rate risk by moving your savings to the UK - you don't know if it'll be better or worse when you want to transfer money back. But I guess it won't be that soon if you want to buy a house, so I think the risk is probably worthwhile.
(I calculated the cost of the transfer by converting €100k into GBP, and then converting the resulting amount back again. That left €99k, so a two-way transfer cost 1% and from that I deduced that a one-way transfer costs roughly 0.5%)