Watching the women's ODI and looking at some numbers for England and Jos Buttler's toss decision. It feels like they always field if they win the toss. However..
When he wins the toss and fields first, England's win % is 47.1%
When he wins the toss and bats first their win % is 100% (that's 2 matches v New Zealand and one v India and Pakistan)
Whey he loses the toss and England field first, the win % is even worse 33.3% (only three matches though)
When they lose the toss and bat first the win % is 61.5%
Overall, batting first England have won 70.6% of matches. When chasing it's 45%.
With all that said, if England win the toss today, I expect them to field first.