I am not sure regarding weather report; but ideally England should pile up or try to pile up 700 here, provided that 450 overs are almost certain.
One reason is that, this wicket is never going to be better & if there is anything for bowlers, it's on day 4/5 & may be early morning. So, ideally, England should bat another 40 overs today, may be bowl 4/5 overs tonight & use the new ball tomorrow morning. Even in case of rain, may be England's best chance to win is by enforcing follow on.
2nd reason is a very thin one, but still it's a chance. England has scored so quickly that, there is actually enough time for PAK to win this Test. If England declares for say 554/7 with 10 sessions to go, PAK can pileup 650 & Yadir can run through England for <250, to set up a target of ~150 in 35 overs. Very little chance, but England indeed lost a Test at Adelaide after declaring got 551/6 & almost lost a recent Test in SAF after posting 625+/8 dec. Probably, few overs were lost, but SAF almost managed to beat England there.