Hmm... if you're Silver with DL now, I seriously doubt you'd come close to reaching the 100k levels on UA or AA for free upgrades.

If your travel happens to be in spurts, you can myabe try to do an AA Platinum challenge, which would give you mid-tier Elite for quite a bit fewer miles than the standard required 50k/year. Down side to the AA program is that even as Plat, your free domestic upgrades will be limited as you only earn so many per miles flown and must pay cash for the others.

US used to give a few transAtlantic upgrades per year for midtier elites-- plus thwey give domestic upgrades for free. Given your desire for UK upgrades, US may be your best route. As a bonus, you can add LH flights IAH-FRA--xxx toward accruing US status.