we are arranging flights to vacation in Vegas from UK in July.

Direct from LHR to LAS is around $1200 round trip but booking a round trip to Vegas from Dublin (instead of London) is only $400.

However, the Dublin - Vegas route means Dublin - London - Vegas and changing planes at London and boarding the same BA / Virgin flight from LHR to Vegas.

Can we book from Dublin but ignore the Dublin - LHR leg and get on at LHR or will the ticket be voided?

thanks