Services you've done end-to-end

Hook of Holland to Moscow
Moscow to Beijing in 1980's

I've done Inverness to London Kings Cross once or twice for free in First Class back in the days of East Coast rewards.

Another origin to destination memorable journey I've done is on 2x221s I'm guessing maybe around the year 2005. Edinburgh to London Euston via Carstairs, Glasgow Central, Kilmarnock, Carlisle, Settle, Preston and down the Trent Valley towards Euston.

Most recently (and topically given the announcement the service is to be split):
Halifax – Hull

Longest:
Manchester Piccadilly – Penzance
Brighton – Manchester Piccadilly

In direct sequence:
Stranraer – Glasgow Central, Glasgow Queen St – Inverness (overnight), Inverness – Thurso

Good stuff:
Inverness – Kyle of Lochalsh
Inverness – Aberdeen
Glasgow Queen St – Oban

Routine Inter City stuff:
Manchester Piccadilly – Euston
Leeds – King’s Cross
Bradford Interchange – King’s Cross (Grand Central)

Northern stuff (a few others are missing because I could never resist jumping off for a pint):
Bradford Forster Sq – Skipton
Bradford Forster Sq – Ilkley
Leeds - Bradford Forster Sq
Leeds – Nottingham
Leeds – Sheffield stoppers both via Castleford and Wakefield Westgate (and fasts via Kirkgate before they were extended through to Lincoln)
Leeds – Skipton
Manchester Piccadilly – Hadfield (yes, via Glossop)
Manchester Piccadilly – Marple Rose Hill
Preston – Ormskirk

Elsewhere:
Ipswich – Lowestoft then Lowestoft - Norwich
Norwich – Sheringham
Norwich – Great Yarmouth
Ipswich - Peterborough
Southend Victoria - Liverpool Street
Portsmouth Harbour – Waterloo
Swansea – Paddington
Slough – Windsor & Eton Central
Twyford – Henley
Plymouth - Gunnislake
Newquay – Par

Historical Oddities:
Manchester Victoria – Bury pre Metrolink
Manchester Victoria – Edinburgh on the multi-portion working, 1977 or 1978
Rochdale – Euston and Euston – Manchester Airport - courtesy of First North Western!
Liverpool St – Harwich International boat train, 1991
Victoria – Dover Marine boat train for the Jetfoil, 1992
Hastings – Ashford when still a DEMU shuttle, 1976
St Pancras - Manchester Piccadlly 1977

For my 9,000th post, I'll share with you some of my longest end-to-end journeys. (Don't know why I hadn't spotted this thread before!):

UK:

Aberdeen-Penzance
Kings Cross-Aberdeen
Kings Cross-Inverness
Euston-Inverness
Euston-Mallaig (changing coaches en route)
Paddington-Penzance
Paddington-Fishguard Harbour (and vv)
Euston-Glasgow Central (via GSW)
Glasgow Central-Nottingham (via GSW & S&C)
Brighton-Manchester Piccadilly

Europe:

Paris-Nice (via classic line & LGV)
Paris-Toulouse
Paris-Perpignan (via LGV)
Valencia-Madrid (in a two car DMU)
Madrid-Lisbon (in another two car DMU)
Paris-Venice (Sleeper)
Paris-Geneva (TGV)
Paris-Milan (TGv & Frecciarossa)
Milan-Lecce (loco-hauled(
Turin-Paris (loco-hauled)
Rome-Siracusa (including train ferry)
Palermo-Rome (loco-hauled)
Hoek van Holland-Copenhag
Frankfurt-Innsbruck
Munich-Rome
Vienna-Zagreb
Ljubljana-Belgrade
Belgrade-Thessaloniki
Thessaloniki-Athens
Oslo-Stavanger
Bergen-Oslo
Oslo-Trondheim
Trondheim-Bodø
Narvik-Stockholm

In a roughly chronological order:
Various other mundane journeys on branch line stubs in the south-east throughout the same period above.

The Thames-Clyde Express in 1974. Glasgow Central to St Pancras via Dumfries, Settle, Leeds and Nottingham.