It's not easy to play in UK, particularly in May. This is not like the 1980s (Or earlier) when teams used to land at Heathrow last week of April & play 1st Test at Edgbaston on 2nd Thursday of June, then 2nd Test at Lord's on 4th Thursday. Bradman scored over thousand FC runs almost every time he toured England, before playing 1st Test. Also, Lankans are playing back to back Test, starting in 8 days gap; normally in ENG, Tests are played in alternate weeks.
This is a very young & inexperienced Lankan team, not sure how many of them had toured England before and they were put into the toughest 2 grounds in North England, in May. I know, these are excuses, BUT, ECB (In fact up to Uganda Cricket Board) is not PCB - so, instead of Southampton & Cardiff, they put Lankans at Leeds & Durham in those Tests in May. On such condition, most teams 'll suffer, may not be to such extent, but it would have been difficult even for AUS or SAF to face this English attack on these context, after 1 weeks preparation, particularly coming from South Asia. Few years back, PAK got all-out under 100, probably 2/3 times in 4 Test - that too in 2nd half of the summer & the very next year an experienced Indian side lost 4-0, with 1 or 2 innings defeats.
I am sure Lankan's 'll put a much better show after 2 more weeks in UK & the summer is into June on a good Lord's track.