Citizen of the UK travelling from there to Amsterdam on 11th April

As Harper explains, you will definitely be allowed back in the the UK if you manage to get there, and the remaining EU will have absolutely no reason to launch a witch hunt for stranded Britons on short visits.

A different worry is that your flight home might be canceled if it turns out that a no-deal Brexit means that EU airlines have no legal right to serve UK airports and vice versa. If your ticket is with an UK airline, this now seems to be unlikely as the EU has passed legislation to allow UK airlines to fly to the EU for a transitional period even after a no-deal Brexit. They're poised to withdraw this quickly if the UK doesn't offer equivalent access to EU airlines, but they'd probably allow at least a few days time for the UK to fix the cock-up if things start to melt down immediately after Brexit.

I'm not aware of any positive corresponding rulemaking in the UK. This gov.uk "guidance" from 7 March states an intention to give reciprocal access, but it is not clear what would need to happen to make that a legal reality, and whether the government is on track for getting those things to happen. It doesn't seem terribly likely that they'll choose this as the opening shot in an all-out trade war, but if your bookings are with a EU airline, you may want to start preparing mentally for possibly being stuck on the continent for several additional days before you can repatriate yourself via sea, tunnel, or a UK carrier.

• The EU has declared that the right of British expats are a matter for the individual member states. The Netherlands are taking measures to give temporary residence permits, so there should be no problem for visitors, either.
• It is conceivable but unlikely that something could happen to air traffic. There are suggestions of temporary permits even in the absence of the Brexit deal, but these are contingent on reciprocity. There could be a tit-for-tat if things turn unfriendly.
  It might be a good idea to have enough money in the bank account/credit card limit to take alternative routes home, e.g. a ferry or the train.

Easy. You can definitely re-enter. One huge feature of being a citizen of country X is that you cannot be refused entry into country X.

So as a UK citizen you are entitled to enter the UK.

If you show up without papers, they can detain you until they confirm your citizenship. However if you have proper documents along, it'll be no trouble at all.