hi what flight search site are those prices on? thanks
If you browse the site you should find multiple methods folks here use to find flights (plus a lot of hints on methods and the flights to choose on the "ex-eu" threads too).
As a 'live' example take a look via thematrix.itasoftware.com for flights to LAS:
Put DUB as "Departing from", LAS as "Destination", click on advanced controls and type LON in both routing codes boxes to route via London airports, click "See calendar of lowest fares" and put, say, 11/01/2018 to choose from 1st November 2018 (and a month later), put 6-13 for "Length of stay" or any range spanning maximum of 7 nights between least and most, change "Cabin" to business class or higher, change "Stops" to up to 1 stop and make sure you check "Allow airport changes" & probably easiest for comparison to change "Currency" to GBP before clicking on "Search".
This brings up a nice calendar month with lowest fares from Dublin to LAS (via London) with stays of between 6 and 13 nights …. if you scroll over a particular date you'll see prices for that day for each number of nights. Let's say you choose/click 13th November for 12 nights (at indicated price of £1368) … this brings up a list of options at that and at higher prices. Now you'll see that the early entries at £1368 are all using BA Business DUB/LHR-VS Upper LGW/LAS -VS Upper LAS/LGW -AerLingus Economy LGW/DUB and because the 'last leg' does NOT involve an airport change your luggage will normally get through checked LAS/DUB. However if you look a bit further down you'll see at £1373 the options for BA Business DUB/LHR-VS Upper LGW/LAS -VS Upper LAS/LGW - BA Business LHR/DUB so you can complete the 'last leg' or not pretty much as you choose as you collect your luggage at LGW on the way back!
To book then click on the £1373 box next to the fare you've chosen and this opens up a more detailed screen with fare rules etcetera. If you then do ctrl-A to select all and ctrl-C to copy then in another browser window open bookwithmatrix.com and do ctrl-V to paste the information in …. bookwithmatrix then checks availability on Flight Network (and others) - don't worry if, in this example, it comes back with a box "Book with flight network at £2924" as real price will show once you click that button and it takes you to their site and comes up with the £1369.36 fare as below

…. note, this was a "safety first" booking where you would have popped out to Dublin on a cheapo fare on the Tuesday and maybe spent the day or afternoon relaxing there, started your first leg on BA back from Dublin to LHR on Tuesday night to overnight at LGW or wherever before grabbing the VS43 the next morning after a small CH visit!
Departs:
Dublin, Ireland (DUB)toDublin, Ireland (DUB)
Trip Duration:42h 00m
British Airways
Flight 829
Departs:
Tue, Nov 13 8:40 PM
Dublin (DUB)
Arrives:
Tue, Nov 13 9:55 PM
London (LHR)
BusinessDuration:1h 15m Connection Time:12h 15m Requires overnight stay
Virgin Atlantic Airways
Flight 43
Departs:
Wed, Nov 14 10:10 AM+1
London (LGW)
Arrives:
Wed, Nov 14 1:00 PM
Las Vegas (LAS)
BusinessDuration:10h 50m Connection Time:290h 30m Requires overnight stay
Virgin Atlantic Airways
Flight 44
Departs:
Mon, Nov 26 3:30 PM+1
Las Vegas (LAS)
Arrives:
Tue, Nov 27 9:25 AM+1
London (LGW)
BusinessDuration: 9h 55m Connection Time:3h 50m
British Airways
Flight 836
Departs:
Tue, Nov 27 1:15 PM
London (LHR)
Arrives:
Tue, Nov 27 2:40 PM
Dublin (DUB)
BusinessDuration:1h 25m
Ticket Price Details
Adult x 1
[+]
Air transportation charges
£1,277.15
[+]
Adult taxes, fees & charges
£92.21
Adult Subtotal£1,369.36
Total:
£1,369.36
Includes Taxes & Fees
Prices quoted are in GBP